We've had threads on Bryan winning in 1896 at 36 - my "Setting Down Root" had him in 1908, clearly a more traditional age by then - and I think someone even had Clay become President in 1812 at 35 once.
So, let's go the opposite end of the spectrum. This is hard because if something happened with a Hillary investigation early or she was just too ill to run, Bernie Sanders might be the nominee anyway, so I couldn't just do a challenge of a candidate winning at 75. I decided to push it to 80.
However, I did give you an out and made it 78. that way, with my other criteria, he could serve a while at 80 and over if you want. 78 means he can serve half his term in his 80s. If he survives of course.
The criteria is to have a President elected and serve at least part of his term in his 80s. It can be a 2nd term but not a 3rd, so no Reagan 3rd term (which was already handled well with his Alzheimer's, anyway.)
No fair having a terror attack be so bad Strom Thurmond becomes President in his late 90s, that rare possibility blows everything so far out of the water we might as well not have a challenge. So, the person has to be elected.
This is in pre-1900 because, while people live longer now, an election without the rigors of campaigning might be easier to do this with. Although, by my definition, the person *could* be a nominee as an emergency after something awful happens to the prospective nominee. And, of course, it could involve a famous person surviving who died earlier OTL. (Theodore Roosevelt if his cousin doesn't run for a 3rd term?)