Biden is certainly old (just a bit more than my age). Too old in my opinion to be in such a position.
But Biden isn't showing any (or very, very few) signs of his age from a mental point. Trump on the other hand seems much more confused than my father was at 85. It's silly to diagnose anything from watching him on TV but it seems to me that a serious degree of mental deterioration is one explanation for the bananas things he says every time he opens his mouth.