Something I noticed in my own elderly relatives and later read some scholarly articles about may apply to Reagan (I say may because I’ve no idea).
Okay, imagine the descent into senile dementia and or Alzheimers (whether they’re the same thing depends on who you’re reading) as a scale from 1 to 10.
Suppose on this scale
1 is the beginning phase. Grandpa is getting really forgetful; he knows his name and address, but he’ll completely forget to take his medicine or that he was supposed to go to the doctor today type stuff, and far more frequently than can be explained by simple age related absentmindedness.
5 is the midway point. Grandpa has good days and bad days. On his good days he recognizes most people and has most of his memories (especially of his youth or things that happened years ago- his short term memory may be shot). On his bad days he sometimes can’t recognize his wife or children and has no idea what year or day it is. (At point 5 he’s incapable of independent living but may be capable of assisted living.)
8 is mostly bad days with occasional moments of lucidity or something like where he recognizes people, but these are now the minority.
10 is the stage where he’s essentially a vegetable who may be able to see or respond to things but has absolutely no recollection of anyone or anything from his old life and must be assisted with all bodily functions.
Okay, most people who go through these stages don’t experience Point 1 for a while then gradually fade into Point 2 then point 3 then point 4, etc… Rather than a gradual deterioration it’s not at all uncommon for Grandpa to go straight from Point 1 to Point 4, then from Point 4 to Point 6, then from Point 6 to Point 8, then to 9 then to 10. The gradual collapse is usually in the latter part of the descent.
The reason is human adaptability. All of us if we lost the use of our right eye would begin to compensate fairly quickly: much wider head turns when backing out of a parking lot would become second nature almost immediately, or instinctively holding a paper you’re reading with your left hand only; likewise if we go for a year with our arm in a cast by the end of that time we’ll know how to tie our shoes/shave/bathe/use a camera/type/etc. with one hand.
Elderly people experiencing the memory loss and dementia of senility/Alzheimers are no different. They’ll figure out coping mechanisms that while they may not mitigate the decline at all they can at least help them keep up appearances. Some of it is by reciting information several times, or speaking the obvious or answering questions that haven’t been asked.* The behavior may seem a little peculiar perhaps but nothing more than just “he’s a bit eccentric”.
However, as the mind wears down they can’t keep up. Essentially, they’ve really been at about a 3 but they’ve been able to give the appearance of being about a 1, but when their mentalia deteriorates one day they wake up and they’re unable to create that illusion of a 1 anymore, and in fact they’re now a 4. It seems to happen overnight but in fact it was more prolonged. Then they start to maintain the appearance of a 4 when in fact they’re a 5 and so on. Then after a while their ability to fake being better goes away and so they decline in appearance at the same rate as they are declining in reality. (I hope this makes sense.)
Anyway, it could (and I’m not declaring it one way or the other) have been this way with Reagan. It’s possible, especially with his background as an actor and as a politician (two professions where even if you have a low grade fever and just learned your daughter’s having an affair with a married man and your campaign manager is being tailed by feds you still have to seem cheerful and in charge when in public) that he was able to keep up the appearance of a man who was vibrant and in charge of his faculties when in fact he was just barely hanging on by his fingertips, and then one day his fingertips slipped (which could have been around 1994 when he made his last public appearance [Tricky Dick’s funeral] and his family made the announcement he had Alzheimers).
*Something I’ve noticed some older people whose minds are beginning to slip is playing “details and connections”; when they tell a story for example there will be FAR more detail than there used to be. Instead of “your grandma and I went to Niagara Falls for our second honeymoon” it’ll be “your grandma and I went to Niagara Fals for our second honeymoon… that was in 1973… June… we stayed at the Honeyfield Lodge, where they had all you can eat Maine lobster for $12.95, and we watched Tony Bennett on The Tonight Show while we were there… there was a blue quilt on the bed, and it was the first time I ever tried an Old Fashioned- bartender’s name was Ernie, he looked like Joe Garagiola” type stuff. I think they know you’re not interested in most of this, but by repeating these things it helps [my theory only I should specify] cement them into their memory, something like a fixative, and by making these connections it helps them stand on firmer ground as other memories fade and disappear altogether.