The majority of people on the autism spectrum are diagnosed while they are young. However, some people are not diagnosed until they are teenagers or adults.

This is largely owing to how subtle the signs of autism can be, as well as the long-held belief that girls are statistically less autistic than boys. Recent results on autism spectrum diseases, on the other hand, have confirmed that autism does not "develop" in teenagers or adults.

Development vs. Diagnosis

Autism is not as developed in older children or adults as it is diagnosed, especially in individuals who have not received an appropriate diagnosis of autism in recent times. Therefore, one of the most important requirements for an autism spectrum disorder diagnosis is the presentation of symptoms that occur before the age of 3 (early childhood in pediatric terms).

When an older child or adult unexpectedly develops problems with their behavior or social communication, it may be due to various causes. However, based on a current and established understanding of autism, this is not autism. Instead, such problems may result from mental illnesses that have been present since early childhood but take years to appear. 

Late-Onset Autism

To understand how autism develops, it is necessary to distinguish between late detection of symptoms and the late emergence of symptoms.

The Diagnostic and Statistical Handbook of Mental Disorders, Fifth Edition (the most recent manual from the American Psychiatric Association) indicates that symptoms must appear during the early developmental period (before the age of three) to meet the criteria for an ASD diagnosis. According to the DSM-5, these symptoms may not manifest fully "until social demands surpass limiting capacity" or because the person has learned coping mechanisms to mask their problems.

High-functioning autism is an example of this, in which an older child (or even an adult) receives a positive diagnosis considerably later than most autistic children. This is not because the individual got autism later in life, but because the symptoms were so mild (and the person lacked the abilities to deal with them) that parents, caregivers, and even medical experts missed the signals. Only with time does the certainty of a positive diagnosis of autism become more likely. 

Diagnosis of Autism In Girls

The "masking" autism symptoms are more pronounced in girls than in boys. Girls have traditionally acted passively in order to avoid being perceived as troublemakers. What appears to be calm and subservient behavior may be the result of undiagnosed autism that the girl (or child) was born with but did not develop.

Because autism symptoms can be so subtle, people who are high-functioning on the autism spectrum may go for years without receiving a formal diagnosis, leading to the mistaken belief that their autism evolved as they grew older.

For example, adults with high-functioning autism may have relatively mild social and communication challenges compared to those with more recognizable symptoms of autism. Unfortunately, many of these adults have been misdiagnosed with attention deficit hyperactivity disorder as a result of this.

Similarly, because their autism does not fit disability stereotypes, many girls with high-functioning autism are misdiagnosed (or not diagnosed). No two cases of autism spectrum condition are the same, and some cases are out of the ordinary.

High-functioning individuals may also be better at hiding the signs of their condition, fitting in better with others, or avoiding being bullied or abused.

Autism Misdiagnoses

Maia Szalavitz, writing for Scientific American, explains how many parents of autistic daughters have been repeatedly told by clinicians that their daughters may not have autism. One example was the story of a mother of two children, both with positive diagnoses of autism spectrum disorder. Even though her daughter's symptoms were far more visible than her son's - a speech delay, repetitive behavior, stringing her toys together, an inability to cope with routine changes and frequent meltdowns - one clinician after another found an alternative explanation for her daughter's symptoms.

Many experts thought the prospect of females developing autism was remote.

Her daughter's symptoms would have qualified her for early intervention at 18 months, but she was diagnosed later at the age of six. She didn't get autism after she was a kid. Her misdiagnosis was persistent.

Her brother, on the other hand, had far fewer symptoms than his sister, but he was diagnosed by the first professional who examined him.

One notable distinction is that many medical professionals are skeptical about the prevalence of autism in girls. As a reason, they are hesitant to make such diagnoses, pointing to other possible causes for female children not being on the autistic spectrum (sometimes simply low self-esteem and, in other cases, even parenting). Significantly, parents are rarely, if ever, told the same things about their son's behavior or diagnosis.

Invisible Patients

Because the paradigm for the conventional autism diagnosis is based on the male behavioral model, there is a trend that girls are not eligible for a positive autism diagnosis. According to a professional neuropsychologist, girls "tend to present their autistic symptoms more discreetly," such as not exhibiting repetitive behavior or having as many meltdowns.

As a result, generations of girls have gone undiagnosed with autism because they do not behave as autistic children should. As a result, these girls have earned the moniker "invisible girls." Only a few of them would be diagnosed as adults, with the belief that their autism would develop as they grew older.

Gender roles may also be a factor. Girls may be better at disguising autism warning symptoms than boys since they are educated and expected to regulate their behavior in public (to a greater extent than guys).

Although girls with autism may exhibit some symptoms, teachers and caregivers are less likely to notice because their attention is often focused on autistic boys who exhibit more disruptive symptoms.

Until a greater understanding of the nature of autism in girls became available, clinical approaches to the development and treatment of autism in females were largely unfocused, according to the primary author of a study published in Molecular Autism and quoted in Stanford Medicine. Much more attention has been given to autism spectrum disorder in boys. 

Help for Adult Patients

Because an adult or teenager can't develop autism, there are also no standard diagnostic criteria or tests for older children and adults with suspected autism spectrum disorder for children under age 6.

For now, clinicians must rely on a series of face-to-face interviews and interactions and extensive consideration of other symptoms that the patient reports. Adults frequently do not receive the autism diagnosis that would be beneficial to them. Clinicians are less likely to suspect autism if they were not thoroughly screened as children, and they are more likely to have a mental or affective condition.

In addition, adults who do not know they have autism are very adept at masking their symptoms. This makes the diagnostic process more difficult for both themselves and their clinicians.

When autism is identified, a physician may need to inquire about the patient's childhood memories with his or her parents. Delays in language, social, and emotional development may be remembered by parents or other caregivers. This information can inform the clinician's diagnosis.

Because autism diagnoses are usually made at a young age, it can be difficult to find a provider who offers or is even willing to offer such a diagnosis for adults. Therefore, when the diagnosis is made, it is likely based on the person's childhood reports and information from others around them.

How Autism Development Works

If a teen or adult can't develop autism, how does autism develop? A 2014 report in the New England Journal of Medicine suggests that brain changes that occur "long before birth" can cause the symptoms of autism spectrum disorder.

Studying children's brains with and without autism, scientists discovered abnormalities in brain regions that control language, social and emotional control in 90% of children with autism. And the abnormalities themselves were formed as a result of a "process" that occurred long before birth."

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

The National Autistic Society Centre for Autism in the United Kingdom stressed the importance of early detection. The earlier patients, parents, and therapists begin treatment, the easier it will be for them to adjust to atypical brain development.

Second, it is out of fear that undetected cases of autism will lead to more misunderstanding that the disability develops with age. This misunderstanding will affect how these people are treated.

Can You Develop Autism?

The consensus is no; autism cannot develop in adolescence or adulthood. However, it is common for autism to be overlooked in girls and people with high-functioning autism at a young age. Because they are not accurately diagnosed, some people may believe that they developed autism as they matured.

In reality, these people have always had autism. Either they were good at masking the symptoms of a disability, or they were high-functioning enough that the telltale signs of autism went unnoticed. The autism was always there; the diagnosis was just delayed.