The Chief Executive only has to make decisions. Everything else is just show business, including the notion that the president “runs the country”. The President gets in line behind an entrenched bureaucracy, and then the elected Congress. If either of those guys can still think and use their wisdom of age properly they qualify. I realize that people think they are electing the King, so they want to inspect the candidate much in the same way that people inspect animals for breeding. But that is nonsense. Anyway, I have seen a list of likely Dem candidates for 2020 and there are plenty of young candidates on it. No guarantee that either Biden or Sanders (or Hillary Clinton for that matter) would make it through the selection process. Then, they would have to beat Donald Trump.