Why, when Trump is old and makes gaffes, too, does the public have less concern about his ability to serve than Joe Biden’s?

Because, as everyone knows, even if they won’t admit it, Trump is simply more vigorous in public. Here is a pretty good contrast.

Trump with a WWE influencer:

Biden at a Juneteenth celebration:

Would Biden be able to pull off what Trump did with the wrestler, which — although it’s not playing Macbeth — requires some performance ability, and would Trump ever seem as out of it as Biden at that event?

The answers are obviously “no” and “no,” and people have noticed.