My son who has Downs is 19. He just lives to go WDW. We do have to use a wheelchair in the parks...he just doesn't have the stamina to walk it. I find it better to rent from an off-site company so we have it all the way from the bus. The walks from the bus can be very long. One of the reasons we love going is because it is the one place he can be like everyone else. Just be prepared for him to really get into things that you might not expect an adult to enjoy. For example, my son LOVES the Disney Channel Rocks show. He likes to do the Kim Possible adventures in Epcot...but i have to help him understand what to do. And he wants to do the Jedi thing but he is simply too big for that. He still loves most of the characters. But he claims that IASW and Dumbo are now lame.
I just let him pretty much set the pace and we have a great time.