The fact that Joe Biden is pretty old is, in my view, largely irrelevant. A good leader is someone who will surround themselves with intelligent, specialised people who know what they are talking about - and more importantly will listen, and has experience in public office. Yes, he will make the crucial decisions and take decisive action where necessary, but it will be on the basis of a willingness to take on board the advice in the first place and make informed decisions. He also has a relatively young and energetic VP who I think will be more than just another 'suit' as many VP's have been - including Biden himself.
It is easy to forget that one of the greatest leaders in a time of extreme difficulty was FDR. He was frail as he was essentially wheelchair bound. It didn`t affect his decision making, his ability to listen to the right people and choose the right people in the first place.