It's funny because Reagan had Alzheimer's. Classic.
I know there are a bunch of crazy liberals (including Reagan's son Ron) who have said that he was showing signs of Alzheimer's/dementia during his second term, but I haven't heard any solid evidence to back that up other than possibly Iran-Contra.
He certainly exhibited signs of early dementia around about his second term. His first debate performance against Mondale was a disaster. It was a bit like James Stockdale's performance in 1992. He was a bit out of it. Now, he recovered afterward for awhile, but after that, times like that where he seemed sort of lost popped up more frequently. While not necessarily full blown Alzheimer's, it does look like he had early dementia beginning in 1984.