I don't know for certain but Disney does seem to be very firm about age regardless of any disabilities. Even on the dining plan if the child is in the adult age group but doesn't eat two bites you must still pay based on age.
As to the 18 YO yes he will count as an adult, however if he requires a companion, charging extra for that companion to stay with him would probably not be appropriate under ADA. Best thing to do is to ask the question.