I can think of a few recent instances when someone who was too old insisted on staying in a position of power. In addition to Biden, Robert Mueller and RBG are the two who come to mind. Unfortunately, we will probably see more of that over the next decade.
Biden has enough delegates to be the party nominee. If he wants to bow out, he can. Probably, the party would then rally around Kamala Harris as the nominee. I say there is a 50% chance that Biden will bow out in the next seven days.