Joe Biden

In general, it is more risky to have a 70+ person as presidential candidate, or as president.

Changes do occur in the early seventies, more in the late seventies, for many people.
Problem is, we often - even usually- don't know which people will experience those changes.

I've observed that Bernie Sanders has exhibited quite a bit of stamina thus far in his daily, rigorous campaigning.
Joe Biden has as well, in what we see of his day-to-day life, including the recent death of his son, Beau.

Disclaimer - I am older than either of these men, and have observed a slowing of energy and other changes in myself.