Actually, Bernie would be 74 if he took office in January, which is pretty borderline in my opinion. If he waited four years, he'd be 78. Not a good idea.
Just an observation, but most leaders seem to be right at or near their peak right up through their early seventies. After that, often by their own admission, they don't have the full mental capacity to do what they used to. Although many appear quite sharp to others, often these people choose to slow down and take fewer responsibilities, if they don't retire outright.
I do agree with you that I hope that Hillary, if elected, chooses only to serve one term. She'll be 73 at the end of the first term, which is right on the borderline of where people's mental acuity starts slowing down.