In the update review of the antiinnatia autism theory I explain why dental amalgam mercury vapor causes autism in infants but not in adults. Namely because beyond a certain age the behaviours associated with the innatons become established as learnt habits, skills, knowledge, as the elaborated connectivity of the brain, and as crystallised intelligence in place of fluid intelligence.
There might be thought to be a valid objection to this in terms of three case reports of autism beginning at later ages, 11, 14, and 31 years.
It is notable that all three cases resulted from herpes encephalitis. In respect of the 31-year-old, he was in a coma for more than 14 days, and from having been a university lecturer was permanently transformed to having a mental age of less than 2 years. We can thus see that it was only in the context of an extreme level of brain damage that the characteristics of autism resulted. So we can interpret his case as being one where even the established habits, skills and so on were wiped out, by a condition substantially more severe than (for instance) mercury poisoning. (Not that mercury poisoning should be considered non-serious itself.)