I wander if there should be such as Adulthood Disintegrative Disorder, a form of very late onset Autism where an older teen or an adult naturally loses their high IQ, social skills, motor skills, memories and might develop psychosis at the age of sixteen or older. But this form of Autism that I am talking about looks related to Dementia.
I remember that Childhood Disintegrative Disorder was in DSM-4.