Why does autism sometimes only visibly appear later in life?

It is important to remember when reading this question and answer is that Autism Spectrum Disorder (ASD) does not develop in adulthood, but symptoms can change for individuals over time because learning occurs.

Symptoms of ASD come in all shapes and forms because this is a spectrum disorder and therefore people present somewhere on a spectrum, often referred to as from low functioning to high functioning.

To answer the question directly, people who present symptoms later in life, as you say, often do present those symptoms in childhood but they aren't noticed by the adults in the child's life until it becomes a real problem for the individual.

As an example, interpersonal skills are learned during childhood. It is perfectly acceptable for children to be at slightly different stages during development with regard to interpersonal skills. Some children will be more advanced then others. Interpersonal skills can be difficult for people with ASD to grasp and so someone with high functioning Autism can take longer to develop these skills, if they are developed at all. So that would potentially not show up until later when the other kids have developed these skills and the child with undiagnosed ASD has not.

Often, 30+ years ago, before we had more advanced treatments and assessments children would 'fall through the cracks' and be labeled as gifted, rather then as having high functioning ASD. This would explain why some people who are highly successful in adulthood are diagnosed in adulthood rather then childhood. People with high functioning ASD often have a savant type qualities, meaning they are very skilled in some specific area. The diagnostic criteria (DSM) has been amended over the years which would account for some adults being diagnosed now, as they may not have qualified before the DSM changed the criteria.

This would be an interesting study to read to understand social interaction among children with high functioning ASD: Bauminger, N., Solomon, M., Aviezer, A. et al. J Abnorm Child Psychol (2008) 36: 135. https://doi.org/10.1007/s10802-007-9156-x

This article discusses changing symptoms (Behavioural) in youth and adults with ASD.

This might be a good book for you to read together a good understanding of ASD in development, which might give you a better understanding as to why people can go undiagnosed until adulthood. Handbook of autism and pervasive developmental disorders, 2nd ed.

In conclusion, There are many reasons that someone might not be diagnosed until later in life, and autism is a spectrum which means people have many different symptoms that range on a vast scale.

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References

Bauminger, N., Solomon, M., Aviezer, A. et al. J Abnorm Child Psychol (2008) 36: 135. https://doi.org/10.1007/s10802-007-9156-x

Shattuck, P.T., Seltzer, M.M., Greenberg, J.S. et al. J Autism Dev Disord (2007) 37: 1735. https://doi.org/10.1007/s10803-006-0307-7

More of an opinion based answer- the symptoms become more distinguishable from kids and adolescent behaviour when you age.

Restless behaviour, stimming behaviour, echolalia etc. can be confusing with typical childish behaviour.

Failing to organise things, fine motor skill problems, making clutters, forgetfulness etc. can be easily confused with adolescent and early adult behaviour.

But when the patient is adult, say 20 or 30 years old, yet having unusually childish behaviour, lack of social understanding, various skill deficits (and given that the patient fail to 'mask' hard enough); the neurodevelopmental anomalous pattern gets some more visibility.