That's hard to call. I know a lot of 80+ year olds that are functioning well.
100 - 150 years ago, I would have said maybe 70 years old for an age limit.
I think it has to be on a case by case basis. As long as the doctor or board making the call isn't politically motivated.
Biden is definitely gone... no matter his politics. Trump on the other hand seems to still be doing OK.
Some may not "like" his politics, but he is still functioning.
Some of the Senators and Reps should be removed. But I can't really comment on which, as I don't see them "performing" that often.
Then there's some that should be removed, no matter what their age, cause they're insane or at least headed that way...
Like anyone calling for riots, etc... we really don't need people like these.

I think we have some problems with our political system. Like trying to oust a judge cause he slept with a college girl, when he was in college. Now that's purely political.
A lot of this me too crap should be done away with as well.
As for McConnell... again, I don't see him speaking all that often so it's hard to say.
I remember a lot of people saying JFK was too young to be a president, but he did a pretty good job.
We need to be careful about making rules or limits that cover everyone. It definitely needs to be on a case by case basis.