Most people who are on the autism spectrum are diagnosed early in childhood. Some, however, don’t receive that diagnosis until they are teenagers or adults.

This is largely due to how subtle the symptoms of autism can be, and that for generations, it was believed that girls were statistically less likely to be autistic than boys. More contemporary understandings of autism spectrum disorder have confirmed that autism cannot “develop” in an adolescent or an adult.

Development vs. Diagnosis

Autism is not so much developed in older children or adults, as much as it is diagnosed, especially in the cases of those who did not receive an appropriate diagnosis of autism when they were younger. One of the key requirements for a diagnosis of an autism spectrum disorder is the presentation of symptoms that appear before the age of 3 (in pediatric terms, early childhood). 

Therefore, if an older child or an adult unexpectedly develops problems with their behavior or social communications, it may be the result of a number of different causes. However, based on current and established understandings of autism, this is not actually autism. Such issues may be the result of mental health conditions that are present since early childhood, but take years to actually show themselves. 

Late-Onset Autism

In understanding how the development of autism works, it is necessary to see the differences between a late recognition of symptoms and a late onset of symptoms.

The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (the most recent one published by the American Psychiatric Association) states that in order to meet the criteria for a diagnosis of autism spectrum disorder, the symptoms have to present in the early developmental period (before the age of 3). DSM-5 acknowledges that these symptoms might not fully present themselves “until social demands exceed limited capacities” or because the person has learned coping skills to cover up their symptoms.

An example of this is high-functioning autism, where an older child (or even an adult) can receive a positive diagnosis much later than most autistic children would. This is not because the autism developed in the person when they were older, but because the symptoms were so subtle (and the person equipped with the ability to cope with those symptoms) that parents, caregivers, and even health care workers might have missed the signs. It is only with time that the certainty of a positive diagnosis for autism becomes more likely.  

Diagnosing Autism in Girls

The phenomenon of “masking” autism symptoms is more pronounced in girls than in boys. Traditionally, girls are more likely to act in very passive ways, to avoid being thought of as troublemakers. What might appear to be quiet and submissive behavior could actually be undiagnosed autism that the girl (or child) was born with and did not develop.

Because the symptoms of autism can be as subtle as they are, this means that people who are high-functioning on the autism spectrum can go for years without an appropriate diagnosis, leading to the belief that they developed their autism as they aged.

For example, adults with high-functioning autism can have relatively mild social and communication challenges when compared to people with more recognizable symptoms of autism. This has led many such adults to be mistakenly diagnosed with attention deficit hyperactivity disorder.

Similarly, many girls with high-functioning autism are also misdiagnosed (or not diagnosed with anything at all) because their autism does not fit the stereotypes of the disability. No two presentations of autism spectrum disorder are exactly the same, and some presentations are outside the norm.

High-functioning individuals can also be better at hiding the signs of their condition, to better fit in with others or to avoid being bullied or abused.

Autism Misdiagnoses

Writing for Scientific American, Maia Szalavitz explains how many parents of autistic daughters were repeatedly told by clinicians that their daughters could not have autism. An example was a story of a mother of two children, both with positive diagnoses for autism spectrum disorder. Even though her daughter’s symptoms were much more obvious than her son’s — a language delay, repetitive behavior, lining up her toys in order, inability to cope with changes in routine, and frequent meltdowns — doctor after doctor would find an alternate explanation for her daughter’s symptoms.

For many clinicians, the chance of a girl developing autism was low enough to be unlikely.

Her daughter’s symptoms would have made her a candidate for early intervention as young as 18 months, but it wasn’t until she was 6 years old that she was finally diagnosed with autism. She did not develop autism past the early childhood phase. She was simply, consistently, misdiagnosed.

Her brother, on the other hand, had symptoms that were far subtler than those of his sister, but he was diagnosed by the first clinician who saw him.

One key difference is that many health care providers are skeptical that autism is common in girls. As a result, they are reluctant to offer such diagnoses, choosing instead to point at other reasons why female children cannot be on the autism spectrum (sometimes blaming simple low self-esteem, and in other cases even placing blame on the parents). Tellingly, parents are rarely, if ever, told the same things about their son’s behavior or diagnosis. 

‘Invisible’ Patients

The trend of girls not being thought of as candidates for a positive autism diagnosis is because the model for the classic autism diagnosis is based on the male model of behavior. A clinical neuropsychologist explains that girls “tend to have a quieter presentation” of their autism symptoms, such as not displaying repetitive behavior or having as many meltdowns.

This led to generations of girls not being diagnosed with autism because they didn’t fit the model of how autistic children were expected to behave. This has led to such girls being known as “the invisible girls.” Only some of them would go on to be diagnosed as adults, leading to the belief that they developed their autism as they grew older. 

Gender roles might also have a part to play. Since girls are taught and expected to control their behavior in public (to a greater degree than boys), they may be more adept at hiding warning signs of autism. Though girls with autism may present with some symptoms, teachers and caregivers are less likely to notice since their attention is often focused on autistic boys who present with more disruptive symptoms.

Quoted in Stanford Medicine, the lead author of a study published in Molecular Autism noted that until this understanding of the nature of autism in girls became better known, clinical approaches to the development and treatment of autism in girls was largely unfocused. Much more attention had been given to working on autism spectrum disorder in boys. 

Helping Adult Patients

Because it is not possible for an adult or a teenager to develop autism, there are also no standard diagnostic criteria or testing for older children and adults with suspected autism spectrum disorder, as there are for children under the age of 6. 

For now, doctors have to rely on a series of in-person interviews and interactions, as well as a broad consideration of other symptoms that the patient reports. In many cases, adults do not get the autism diagnosis that would help them. If they were not properly screened as children, clinicians are unlikely to suspect autism and will instead lean toward a mental health or affective disorder.

Additionally, adults who do not know they have autism become very adept at masking their symptoms. This complicates the diagnostic process both for themselves and their clinicians. 

If autism is suspected, a doctor might have to ask the patient’s parents about their memories of the patient as a child. Parents or other caregivers may recall delays and problems with language, social, and emotional development. This information can factor into the clinician’s diagnosis.

Since autism diagnoses are typically made when children are quite young, it can be difficult to find a provider who is adept or even willing to offer such a diagnosis for adults. If the diagnosis is given, it will likely rely on the person’s accounts from childhood as well as information from others close to them.

How the Development of Autism Works

If it is not possible for a teenager or an adult to develop autism, how does autism actually develop? A 2014 report in the New England Journal of Medicine suggests that brain changes that take place “long before birth” might be what causes the symptoms of autism spectrum disorder.

Looking at the brains of children with autism and those without, scientists discovered abnormalities in the brain regions that control language, social, and emotional control in 90% of the children with autism. And the abnormalities themselves were formed as the result of “a process occurring long before birth.” 

In terms of timing, these changes occur in the cortex (the part of the brain most responsible for memory and learning) around the second trimester of pregnancy. Speaking to NPR, one of the authors of the study explained that “something must have gone wrong at or before” the second trimester. 

Commenting on the study, the director of the National Autistic Society Centre for Autism in the United Kingdom stressed the importance of early detection. This is primarily because the earlier the intervention, the easier it will be for patients, parents, and therapists to compensate for the problematic developments in the brain.

Secondarily, it is for fear that undetected cases of autism will lead to more misconceptions that the disability develops as some people age. This misconception will affect how these people are treated. 

Can You Develop Autism?

The consensus is no, autism cannot develop in adolescence or adulthood. It is, however, common for autism to be missed among girls and people with high-functioning autism when they are young. Because they aren’t accurately diagnosed, it might lead to some people believing they developed autism as they matured.

In reality, these people always had autism. They were either good at masking the symptoms of the disability, or they were high-functioning enough that the telltale signs of autism evaded notice. The autism was always present; the diagnosis was just delayed.