I don't disagree on your last point, but age is a factor that I don't think matters as much as you imply at first. In the sense that different people are affected in different ways. A good president or politician won't be defined by their age. You know who is older than Biden? Sanders, and he might be one of the most lucid and seemingly good politicians all around (but I'm not in the USA so my understanding is limited, at least what I have heard of him is extremely positive).

That's not to say that I'd rather have younger people involved in important roles in politics, there's too many that should be completely retired, and Biden is one of them. But age is not the main issue.

Dementia isn't a deal breaker. Neither is being paralyzed. You could run for president as a catatonic coma patient if you wanted.

A narcissist would never resign unless it's in their own greatest self interest.

Which is why he didn't run again.

Your senile leader did.

Trump definitely has dementia. He rambled about medians during his send troops to the capital press conference. https://youtube.com/shorts/pY06vb1Cxl0

No he doesn't.