Probably because it would be seen as ageism. One would think that the population would be able to discern the fitness of a candidate. That has not been the case as evidenced by McConnell and Biden. Say what you will about Trump’s unsuitability for office but he hasn’t shown the common signs of age related senility.
It's difficult to say, he's been spouting so much inane babble and utterly nonsensical drivel for years now that it's hard to say whether it's due to senility or not.
I guess you're right, though. He doesn't appear actually infirm like Biden is. I think I'd be less mortified if we COULD blame senility for everything he's said, but I'd concede that point.
why not just make it impossible to hold office when you've reached pension age?
Because politics is an arena where wisdom and experience are paramount, and these attributes accumulate with age.
An airplane pilot needs fast reliable reflexes for obvious reasons, but a political leader does not make quick independent decisions without input from a team of knowledgeable advisors, and physical prowess is almost irrelevant.
And besides, there is a primed successor already installed and ready.
Ehh, I dunno, man. Sure, wisdom comes with age, but even there, there is a definitive prime point, and it's not the second before you die. Plus, a wise leader definitely needs to be able to see the bigger picture, and all facets involved; a difficult taks for even the best of us, yet absolutely paramount for a leader. These guys quite literally have difficulty finishing the sentence they are in. I honestly think this level of decrepity in a position of power, let alone one in the most powerful country on earth, is indefensible.
I also don't understand the ageist thing. I work at a university, and when you reach pension age, you're out, no question about it. It's possible to stay on as a guest sometimes, in an informal capacity, and some keep doing that until basically their deaths. But nothing official, and certainly not in any position of power. My mother worked at the council over in Amsterdam, it's the same there apparently. And sure, some people are sharp as a knife even at 90, but you need to draw the line somewhere. The alternative is worse.