Autism Goes Undiagnosed In About 90% Of Over-40s. I Asked Experts How To Spot It

You might have heard that autism diagnoses are on the rise in the UK. Some even argue it’s being “overdiagnosed”. 

But the National Autistic Society says that about 750,000 autistic adults in the UK remain undiagnosed. And a 2025 review estimated that 89% of autistic adults over 40 remain undiagnosed, with that figure rising to 96% for over-60s.

Here, HuffPost UK spoke to the review’s author, research fellow and co-leader of the ReSpect Lab, Dr Gavin Stewart, about why those numbers might be so how, and how autism might show up among older adults. 

Why are these numbers so high? 

Dr Stewart said that in his research, an overwhelming majority of autistic people aged 40 and older remained undiagnosed. 

He explained that there are “many reasons” for that.

“First, the diagnostic criteria for autism have greatly evolved over the past several decades. It was originally conceptualised with very narrow diagnostic criteria,” he explained.

“This meant that it was a rare condition, affecting around 1 in 3,000 people in the 1960s.”

But as our understanding of autism expanded, he said, so too did diagnoses; autism is now recognised as a “spectrum affecting around 1 in 100 to 1 in 33 people.

“This means that many middle-aged and older people were likely overlooked when they were young as they did not fit the narrow diagnostic criteria being used at the time.” 

Some members of this group, however, can go on to get a diagnosis under the new terms, he added. 

Why does it matter if people stay undiagnosed into older age? 

Having autism without realising it can affect people’s lives in many ways, the study author told us. 

“For some, it might mean that they are not able to access help and support that would be available to them if they did have a diagnosis. For example, autistic people often benefit from additional support while as a child in school, or as an adult when in higher education or in employment.” 

Their physical health might be affected, too. 

“Autistic people often have more complex health support needs, including age-related health conditions as they get older. While accommodations can be made to make healthcare more accessible for autistic people, if the person has not been recognised,” Dr Stewart said. 

And a lack of tailored support can take its toll on autistic people’s mental health, too.

“They are more likely to become socially isolated and have poorer well-being, which in turn can lead them to be more susceptible to periods of crisis like suicide.” 

What are some signs of undiagnosed autism in over-40s? 

Some signs of autism are “common” among both diagnosed and undiagnosed autistic people, said Dr Stewart. 

These can include: 

1) Social difficulties

“In social situations, an autistic person might find it hard to read what other people are thinking or feeling, and they might struggle to tell when someone is joking or being sarcastic.

“Making and maintaining friendships can feel effortful, and people might
perceive them as being quite blunt or uninterested without meaning to. This might make them feel quite anxious and uncertain in social situations, meaning they prefer to be on their own.” 

2) Very consistent and rigid routines

“In daily routines, an autistic person might have a strong preference for sameness, so doing things the same way every time, and when things change, it can be quite distressing,” said Dr Stewart. 

3) Special interests

“They may also have very focused interests, which can be beneficial for
some but not always.” 

4) Becoming overwhelmed and/or overstimulated often

“They may also find certain sensory environments quite
overwhelming, and notice details that many other people do not, like the way a room smells or the buzzing of overhead lights.” 

Dr Stewart noted that some autistic people, especially women, will have learned to “mask”, meaning they “have learned to hide parts of themselves to fit in
better, but this is an effortful process and can have an impact on their wellbeing.”

What if I think I have autism? 

Dr Stewart advised speaking to a healthcare professional, like a GP, if you think you might have autism and want to explore an assessment.

“There are also charities and organisations, like the National Autistic Society in the UK, who can provide information about accessing autism assessments as an adult,” he added. 

You can reach out to the National Autistic Society through this link.