Diagnosed at Last: The Autistic Adults Identified Decades Late

Autism is about as common among adults as among children, but diagnosis is not. In England in 2018, around 1 in 34 children aged 10–14 had an autism diagnosis in their GP record, against around 1 in 6,000 people aged 70 and over — and researchers estimate that more than nine in ten autistic people over 50 may never have been diagnosed^[1].

Many of the people behind these figures grew up when autism was thought to be rare, to belong to childhood and to look one particular way. They went to school, worked, raised families and sought help for exhaustion, anxiety or low mood, often without anyone putting a name to how they experienced the world. Now, in their 30s, 40s, 50s and beyond, more of them are being identified. This article sets out what the research shows about that generation, and what changes — and what too often does not — once the answer finally comes.

Autism is as common in adults, but diagnoses fall away with age

When England's Adult Psychiatric Morbidity Survey clinically assessed a random sample of adults living in private households in 2007, it estimated that 9.8 per 1,000 had autism, and prevalence was not related to age^[2]. The adults it identified, the authors concluded, "tend to be unrecognized"^[2]. Adding a case register of adults with intellectual disability put the prevalence among adults of all ages in England at about 11 per 1,000^[3]. Lai and Baron-Cohen, writing in The Lancet Psychiatry, linked the growing importance of first diagnosis in adulthood to greater awareness, broader criteria and the idea of a spectrum — and to a "lost generation" of people previously excluded from a diagnosis^[4].

Recorded diagnoses tell a very different story. Among 602,433 people registered with English GP practices, 2.94% of 10- to 14-year-olds had an autism diagnosis at the end of 2018, compared with 0.02% of people aged 70 and over^[1]. Among males, 4.36% of 10- to 14-year-olds had a diagnosis, but 0.54% of men in their 30s and 0.18% of men in their 50s^[1].

The same slope appears elsewhere. In electronic health records from the 12 US health systems of the Mental Health Research Network, covering 12.3 million members in 2022, the share with an autism diagnosis recorded that year was 30.3 per 1,000 at ages 5–8, 3.7 per 1,000 at ages 26–34 and 0.6 per 1,000 at ages 45–54^[5]. Australia's 2022 disability survey, which relies on self-report, found autism in 3.1% of people under 25 but 0.3% of those aged 25 and over^[6]. Taken alone, a fall with age could mean autism is rarer in older people. But the English community survey found no such fall, so the likelier reading is that many older autistic people have simply never been assessed.

How many adults may still be undiagnosed

O'Nions and colleagues projected true prevalence from community studies and from the diagnosis rates seen in children in their data, set it against recorded diagnoses, and scaled the result to England's population to estimate how many people have been missed. They are careful to call the results exploratory projections; the figures that follow are modelled estimates, not counts. As of 2018 they estimated that around 463,500 people had been diagnosed and that between 435,700 and 1,197,300 more may be autistic but undiagnosed — 59–72% of all autistic people in England^[1]. The gap widens with age: 52–75% of autistic people aged 20–49 (152,900 to 489,900 people) and 92–96% of those aged 50 and over (251,100 to 591,600 people) may be undiagnosed — a combined midpoint of roughly 750,000 undiagnosed autistic adults aged 20 and over^[1].

The authors note that undiagnosed adults probably over-represent people with fewer support needs, that the projections do not adjust for premature mortality, and that there could be even more undiagnosed autistic women than estimated^[1]. Being missed is not confined to people with fewer support needs: they suggest 55% of autistic people with an intellectual disability in England, nearly 100,000 people, may be undiagnosed^[1].

Adult diagnoses are rising fastest, in several countries

More of this generation are now being identified. In UK primary care records, recorded incidence of new autism diagnoses rose 787% between 1998 and 2018, with the greatest rises among adults^[7]. In the English GP study, annual new diagnoses rose six-fold between 2000 and 2018, yet the gap remains wide: in 2018, about 1 in 250 children aged 5–9 were newly diagnosed, against 1 in 4,000 people aged 20–49 and 1 in 18,000 aged 50 and over, and among 84,000 people aged 70 or older no new diagnoses were recorded at all^[1].

In the US health-records study, the overall diagnosis rate rose 175% between 2011 and 2022, from 2.3 to 6.3 per 1,000, with the greatest relative increase among 26- to 34-year-olds, at about 450%^[5]. Among adults, the increase was larger for women than men (315% against 215%)^[5]. Earlier Danish register data showed incidence rising from 9.0 to 38.6 per 100,000 person-years between 1995 and 2010, most markedly among females, adolescents and adults^[8].

These studies use different measures, periods and populations, so the figures are not directly comparable. Most point the same way; the Australian survey found little change among 25- to 39-year-olds.

Why the rise? The UK study's authors say it could reflect growth in prevalence but more likely reflects increased reporting and application of the diagnosis, and read the rises among adults and females as a sign of greater recognition^[7]. The finding that adult prevalence does not vary with age is consistent with no increase in true prevalence^[2], and O'Nions and colleagues note that community studies indicate true prevalence has been stable over the last 70–80 years^[1]. That is the prevailing explanation rather than a settled fact, but it points one way: much of the rise is people who were always autistic being recognised. Women and girls are a large part of that story; see our article on autistic women and girls.

Many reach diagnosis after years in mental health services

For many adults, the autism assessment is not their first contact with psychiatry but the latest in a long series. Among 161 adults first diagnosed at two university centres in Italy (median age 23), 77.6% had previously been in contact with mental health services and 66.5% had already received at least one other psychiatric diagnosis^[9]. Their first mental health evaluation came at a median age of 13, a median 11 years before their autism diagnosis^[9]. The most frequent earlier diagnosis was intellectual disability — not confirmed on standardised testing in 9 of the 35 people given it — followed by psychotic disorders (16.1%) and personality disorders (14.9%)^[9].

An earlier diagnosis is not necessarily a wrong one: anxiety and depression are common among autistic people and deserve care in their own right (see our article on anxiety). But a later diagnosis appears to go with a heavier mental health history. In the SPARK research cohort of 4,657 autistic adults, those diagnosed at 21 or older had 2.71 times the odds of reporting psychiatric conditions compared with those diagnosed in childhood^[10]. In an online survey of 420 adults, autistic and possibly autistic participants were more likely than non-autistic participants to report mental health diagnoses but less likely to agree with them, often feeling that professionals had confused autistic characteristics with mental health conditions^[11]. In an interview study of nine adults diagnosed after 50, participants had been treated for anxiety and depression, and described growing up feeling isolated and "alien"^[12].

That history can be serious. At a specialist adult diagnostic clinic in England, 66% of 367 adults newly diagnosed with Asperger's syndrome between 2004 and 2013 reported having had suicidal thoughts at some point in their lives, with depression identified as an important potential risk factor^[13]. The authors pointed to risk factors such as social isolation and unemployment, and called for service planning and support to reduce risk^[13] — things services can change. Suicidal thoughts can pass, and help is available. We look at this more fully in our article on autism and suicide.

Diagnosis brings self-understanding, but support rarely follows

The evidence on what diagnosis changes is mostly qualitative and from small samples, but it is consistent. In interviews with 11 women diagnosed after 40, several described moving from being self-critical to self-compassionate, with a greater sense of agency and more acceptance of themselves — though adjusting so late in life was painful^[14]. The nine people diagnosed after 50 saw diagnosis as a positive step that allowed a reconfiguration of self and an appreciation of their own needs^[12]. A scoping review found that adults often have strong emotions after being diagnosed^[15].

The first scale built to measure this was tested in Australia with 92 autistic adolescents and adults, diagnosed at 30 on average. Diagnosis scored as helpful for understanding and accepting themselves, but neutral for being understood by others or getting support from services^[16]. In a survey of 151 autistic adults, more time since diagnosis was associated with less dissatisfaction with their autistic identity, and the authors' analysis of open-ended answers pointed to an emotional post-diagnostic adjustment process^[17].

That neutral score on services is the heart of the problem. Across 11 European countries in 2017, the ASDEU survey asked people diagnosed at 18 or older which recommended elements of post-diagnostic support they had received: 67% of the autistic adults who answered had received none^[18]. A UK survey of 128 adults found satisfaction with the diagnostic process split — 40% dissatisfied, 47% satisfied — with post-diagnostic support among the factors that predicted it^[19]. A 2025 review of UK research on adults diagnosed in adulthood without an intellectual disability found that post-diagnostic support mostly means information and signposting, while autistic adults want psychoeducation and peer support and may prefer autistic-led support^[20].

Adults face particular barriers to being assessed

Assessing adults is harder for practical, developmental and clinical reasons: there may be no one to give a developmental history, many adults have learnt camouflaging strategies, and co-occurring conditions are common^[4]. The route in can be unclear and different for everyone^[15]. In a survey of 665 people who self-identified as autistic or were diagnosed in adulthood, fear of not being believed by professionals was the most common and most severe barrier to seeking a formal diagnosis^[21]. O'Nions and colleagues suggest some professionals may hesitate to raise autism at all, given uncertainty about waiting times and post-diagnostic support^[1]. Where assessment is available, the wait is often long; see our article on assessment waits around the world.

What the evidence asks of us

• Plan adult diagnostic services for the need that exists, not the demand already recorded. The authors of the English study call for urgent improvement in access to adult autism diagnostic services^[1].
• Look for autism in mental health services. Most late-diagnosed adults in the Italian study had already been in contact with mental health services^[9]. NICE guidance for England lists previous or current contact with mental health services, alongside persistent social or communication difficulties or repetitive behaviours, as grounds to consider an assessment^[22]. Researchers call autism training for mental health professionals imperative^[10].
• Make post-diagnostic support part of the pathway. NICE recommends a care plan based on the assessment and a "health passport" setting out a person's care and support needs^[22]. Autistic adults have asked for psychoeducation, peer support and autistic-led options^[20].
• Reach the people most likely to be missed. NICE asks local care pathways to promote access for all autistic adults, naming women, older people and people with learning disabilities among others^[22].
• Fund research on life after a late diagnosis. Reviewers have identified gaps on diagnosing adults with intellectual disability, on differences between early- and late-diagnosed adults, and on support after diagnosis^[15].

None of this asks autistic adults to be anything other than who they have always been. It asks services to catch up with them.

Language: this piece uses identity-first language ("autistic people"), the preference of most autistic-led organisations, and uses clinical terms such as "autism spectrum disorder" or "Asperger's syndrome" only when naming a study or diagnostic category.

Why we're publishing this

Health Insurance UK is a commercial health-insurance resource, not a charity and not a campaign. We compiled this evidence because autistic adults diagnosed late, and the organisations supporting them, deserve the research in one place, cleanly sourced and ready to put in front of a commissioner, an MP or a journalist. The case belongs to the people living it; our part is to say plainly what the data show. This article is part of our Autism & Health research library.

How to read this data

GP and health-system records (England, UK, US) count recorded diagnoses, not how many people are autistic. The Danish figures come from a national psychiatric register and the Australian figures from a self-report household survey. The English estimates of undiagnosed people are exploratory projections, best read as ranges. The 2007 Adult Psychiatric Morbidity Survey figures come from clinical assessment of a household sample and have wide confidence intervals. Studies of mental health before diagnosis use clinic samples, volunteer cohorts or online surveys, which may not represent all autistic adults, and the ASDEU post-diagnostic figure comes from an online survey circulated through the network's partners, not a representative sample. Evidence on what diagnosis changes is largely qualitative, and some surveys are more than a decade old. There is also no agreed definition of a "late" diagnosis: a 2025 review of 420 studies found only 34.7% gave a clear threshold, with cut-offs from 2 to 55 years^[23]. Most adult evidence comes from England, elsewhere in Europe, the US and Australia; data from other regions are scarce.

Use this data

Free to cite with attribution to Health Insurance UK. Charities, campaigners and journalists are welcome to reuse these figures and charts. For the underlying figures as a spreadsheet, get in touch.

Important: This article summarises published research for general information. It is not medical advice and cannot be used to diagnose, treat or predict anything about an individual. If you have concerns about your own or someone else's health, please speak to a qualified health professional.

Sources

1. O'Nions E, Petersen I, Buckman JEJ, et al. Autism in England: assessing underdiagnosis in a population-based cohort study of prospectively collected primary care data. The Lancet Regional Health – Europe, 2023. pmc.ncbi.nlm.nih.gov/articles/PMC10114511
2. Brugha TS, McManus S, Bankart J, et al. Epidemiology of autism spectrum disorders in adults in the community in England. Archives of General Psychiatry, 2011. europepmc.org/article/MED/21536975
3. Brugha TS, Spiers N, Bankart J, et al. Epidemiology of autism in adults across age groups and ability levels. British Journal of Psychiatry, 2016. europepmc.org/article/MED/27388569
4. Lai MC, Baron-Cohen S. Identifying the lost generation of adults with autism spectrum conditions. The Lancet Psychiatry, 2015. europepmc.org/article/MED/26544750
5. Grosvenor LP, Croen LA, Lynch FL, et al. Autism diagnosis among US children and adults, 2011–2022. JAMA Network Open, 2024. europepmc.org/article/MED/39476234
6. Australian Bureau of Statistics. Autism in Australia, 2022 (Survey of Disability, Ageing and Carers). ABS, 2024. abs.gov.au/articles/autism-australia-2022
7. Russell G, Stapley S, Newlove-Delgado T, et al. Time trends in autism diagnosis over 20 years: a UK population-based cohort study. Journal of Child Psychology and Psychiatry, 2022. europepmc.org/article/MED/34414570
8. Jensen CM, Steinhausen HC, Lauritsen MB. Time trends over 16 years in incidence-rates of autism spectrum disorders across the lifespan based on nationwide Danish register data. Journal of Autism and Developmental Disorders, 2014. europepmc.org/article/MED/24554161
9. Fusar-Poli L, Brondino N, Politi P, Aguglia E. Missed diagnoses and misdiagnoses of adults with autism spectrum disorder. European Archives of Psychiatry and Clinical Neuroscience, 2022. europepmc.org/article/MED/32892291
10. Jadav N, Bal VH. Associations between co-occurring conditions and age of autism diagnosis: implications for mental health training and adult autism research. Autism Research, 2022. europepmc.org/article/MED/36054777
11. Au-Yeung SK, Bradley L, Robertson AE, et al. Experience of mental health diagnosis and perceived misdiagnosis in autistic, possibly autistic and non-autistic adults. Autism, 2019. europepmc.org/article/MED/30547677
12. Stagg SD, Belcher H. Living with autism without knowing: receiving a diagnosis in later life. Health Psychology and Behavioral Medicine, 2019. europepmc.org/article/MED/34040855
13. Cassidy S, Bradley P, Robinson J, et al. Suicidal ideation and suicide plans or attempts in adults with Asperger's syndrome attending a specialist diagnostic clinic: a clinical cohort study. The Lancet Psychiatry, 2014. europepmc.org/article/MED/26360578
14. Leedham A, Thompson AR, Smith R, Freeth M. 'I was exhausted trying to figure it out': the experiences of females receiving an autism diagnosis in middle to late adulthood. Autism, 2020. europepmc.org/article/MED/31144507
15. Huang Y, Arnold SR, Foley KR, Trollor JN. Diagnosis of autism in adulthood: a scoping review. Autism, 2020. europepmc.org/article/MED/32106698
16. Arnold SRC, Huang Y, Hwang YIJ, et al. “The single most important thing that has happened to me in my life”: development of the Impact of Diagnosis Scale – Preliminary Revision. Autism in Adulthood, 2020. europepmc.org/article/MED/36600983
17. Corden K, Brewer R, Cage E. Personal identity after an autism diagnosis: relationships with self-esteem, mental wellbeing, and diagnostic timing. Frontiers in Psychology, 2021. europepmc.org/article/MED/34393933
18. Scattoni ML, Micai M, Ciaramella A, et al. Real-world experiences in autistic adult diagnostic services and post-diagnostic support and alignment with services guidelines: results from the ASDEU study. Journal of Autism and Developmental Disorders, 2021. europepmc.org/article/MED/33502713
19. Jones L, Goddard L, Hill EL, Henry LA, Crane L. Experiences of receiving a diagnosis of autism spectrum disorder: a survey of adults in the United Kingdom. Journal of Autism and Developmental Disorders, 2014. europepmc.org/article/MED/24915932
20. Norris JE, Harvey R, Hull L. Post-diagnostic support for adults diagnosed with autism in adulthood in the UK: a systematic review with narrative synthesis. Autism, 2025. europepmc.org/article/MED/39253875
21. Lewis LF. A mixed methods study of barriers to formal diagnosis of autism spectrum disorder in adults. Journal of Autism and Developmental Disorders, 2017. europepmc.org/article/MED/28516422
22. National Institute for Health and Care Excellence. Autism spectrum disorder in adults: diagnosis and management (CG142), Recommendations. NICE, 2012 (updated 2021). nice.org.uk/guidance/cg142/chapter/Recommendations
23. Russell AS, McFayden TC, McAllister M, et al. Who, when, where, and why: a systematic review of “late diagnosis” in autism. Autism Research, 2025. europepmc.org/article/MED/39579014