I'm not sure ageism is applicable when it comes to being the most powerful man in the world. He's on a mission to prove that he's coherent, as the debate did nothing to inspire people to vote for him.
This is not about assuring the base comes out to vote. This is about getting those who really don't care one way or the other to come out and vote. Kids, in particular.
A stammering old man doesn't inspire college students to vote. They're still living a pretty sheltered life.
Ironic, since Bernie the curmudgeon inspired the kids.