Right - I guess technically Biden would be eligible, but for goodness sake, I think even he would admit at that point that it wouldn't be the best move for the party for him to run at age 85.
I'm a firm believer in meritocracy and equal opportunity, so if a candidate can perform the duties of the office then why does being older by itself matter? That's ageism.
Not accounting for death from old age making them physically ineligible, since that's also a very real concern at this point for both.