The central criteria for choosing a president should be their choices and moral compass, a strong voice and leader, the ability to turn a movement toward generating money for the candidate and a finger on the pulse of what’s actually going on.
It also has a lot to do with it when the current 72 year old President shows -- at the very least, arguable -- signs of cognitive decline, and the oldest President ever also showed similar signs towards the end of his term. There are exceptions, but these cases are representative of the typical case, not the exceptional case [1]
Stress is a factor in the above.
Nuclear weapon deployment (not even war) remains one of the greatest threats to civilization and would likely impact another (climate change). The U.S. President must be able to make a fast & sharp decision on this matter should the need arise (though I have to say, Bernie seems likely to make the right choice, which is always "do not launch")
[1] “…most abilities tend to peak in early midlife, plateau until the late fifties or sixties, and then show decline, initially at a slow pace, but accelerating as the late seventies are reached.” (Schaie, 1989).