There's an old post here that tackles the medieval aspect.

Antiquity presents a different set of difficulties. The Greeks didn't really have an understanding of mental disability, but they did understand that some people were different from others. Interestingly, the Greeks didn't draw the lines between mental disability and physical disability the way that we do. Deafness, for example, was treated more like a mental disability than a physical one.

Down Syndrome is a little easier to address than autism, though the answer isn't exactly a happy one. The ancient Greeks practiced exposure of infants and Down Syndrome is recognizable at birth. Generally, obvious physical defects were treated with a kind of religious reverence, the idea being that children so obviously marked were a sign of some flaw in the parents or in the city. We have an abundance of evidence to suggest that children with defects were exposed, but it was almost always a case-by-case decision (except in Sparta). Families could choose to raise a child with Down Syndrome, though child mortality rates were staggering in the ancient world and Down Syndrome comes with a host of other complications that might impact survival. We don't have any remains of adults with Down Syndrome from ancient Greece, but that doesn't mean there weren't any. In fact, the repetitive nature of agrarian labor and the limitations on tools meant that it was probably relatively easy for individuals with certain forms of mental disability to function in their community.

Autism is harder to pinpoint since the term refers to a range of issues caused by different factors. Since it isn't immediately obvious at birth, it isn't likely that autistic children were exposed for their disability. However, ancients lived much closer to starvation and had much less free time than we do. A severely disabled child was probably not likely to survive and even if parents wanted to help, it's unlikely that they would have had the resources to do so. If it was a mild form of autism, then they're likely to have been able to function and probably wouldn't have even attracted much interest.

There isn't a ton written on the subject. The Blackwell companion to Science, Technology, and Medicine in Ancient Greece and Rome discusses it briefly and demonstrates the issues of evidence. Gershon Berkson at UIC has published a couple of articles in the journal Mental Retardation (I can't believe that's a real journal name) about mental disability in the ancient world, particularly Down Syndrome. Obviously not a historical journal, but it still seems to be a solid argument.