Beyond the fact your brain becomes feeble with age

Again, not a fact. People can and do live beyond 100 without losing any mental faculties. What you mean to say is that, at the current time, as humans age, there is a high probability of them developing illnesses that result in mental degradation. That's not the same as saying "it is a fact that your brain becomes feeble with age".

there’s also the practical fact that there are people in congress who haven’t set foot inside a classroom since the fucking Eisenhower administration

I agree, that's much more relevant.

Here’s a question for ya: Should high school diplomas or college degrees expire?

Maybe, maybe not. Either way, if the goal is to maintain a democratic system that isn't designed to induce bias or favoritism of any class over another, then level of education should never be used as a legal requirement to run for political office. That is called an aristocracy. That's not to say the job doesn't have any minimum requirements; the voters are the ones interviewing and hiring for the position, and if education is important to them, they should prioritize it in the voting booth.

IMO if a democracy fails because the voters are too stupid, then it just wasn't meant to be.