aside from the physical issues you mentioned (biden and bernie would literally be older than life expectancy in the usa lol)...the party should be changing with the times and have a 'face' that represents somewhere in the range of the majority of its voters.

the median age of voters overall in presidential elections is 48. democrats disproportionately rely on a good amount of voters even younger than that in order to win (compared to repubs). its increasingly harder to pull those voters in when your candidate is increasingly older (which is why dems historically vote in a prez that is younger than his gop predecessor).

plus older career politicians tend to accumulate a number of votes in the past that run counter to the progressing vision of the current party mainstream (crime bill, gay marriage, etc) and increases their political liability. even progressive bernie voted for the crime bill.