If Team Biden can't allay the fears he is too old, voters will be implicitly pricing in Harris taking his office during the next term anyway. So you either comprehensively demonstrate the "Biden old" stuff is bogus, or if you can't (because true) or can't (because incompetent), get out of the way.
Or to put it another way: every day that Team Biden isn't actually ending the "Biden old" conversation, they are running Harris as the principal at the top-of-the-ticket anyway, but only taking the costs and not taking advantage of the opportunities that come with that
