there are some pretty important differences between biden and jeb too. biden is much more popular with dems than jeb was with pubs in 2015. and by 2015 gwb was pretty unpopular w pubs, while obama is still v popular w. dems. tea party/house freedom caucus voters were a bigger share of 'pubs than dsa/justice dems are of dems, etc.
oth, jeb wasn't a super old man. if biden loses (and i think his chances of winning are well under 50%), i think it will be because voters decide he's too old, not because they reject him politically.