There is an uncomfortable assumption that all autistics are to a degree also ADD/ADHD, which is not a firm comorbidity. There is also a presumption that all autistics are behaviorally unstable. That is also not a firm given. Many autistic children, while delayed in their communications skills and sadly lacking in social skills, still manage to observe the basic rules of a classroom without acting up or acting out.
My son's onset was regressive. He was developmentally normal until roughly 22 months. At that point, he gradually began to stop speaking, developed repetitive behaviors and eventually exhibited other early signs of autism. The onset was gradual enough that he was not formally diagnosed until the age of 4, at which point we entered him into an appropriate early intervention program.
What I find somewhat disturbing is the notion that autistic children somehow come from homes where there is a broken adult either present or waiting to happen. That is not always the case, and the broad brush serves only to obscure the fine details that make this disorder a spectrum disorder. Every child is unique and requires a highly individualized approach to ensure that he or she develops to the fullest potential. Students in my son's program have gone on to college. Others will never be able to write their own name. Yet each one has, in all the years I have known them, never been even remotely aggressive, violent or used inappropriate language. If anything, they could serve as behavioral role models for my neurotypical daughter's classmates.