No. That's age discrimination. If you're concerned that a person could be suffering from mental degradation, require annual testing for it. I know folks in their 90's who are better critical thinkers than a lot of 20-somethings.
The problem we have is not that a bunch of old people run the country. It's that a bunch of young people put them there because they were the only real choices they had. Fix the two-party system first by employing ranked-choice voting. That will break the stranglehold that Republicans and Democrats have on the US political system.
Where did I ever say that? Age discrimination is age discrimination. Either you're qualified for the job or not, independent of your age. It seems like OPs question is a one-size-fits-all reaction to the geriatric choices forced upon us by the two party system. The real solution is to open the system up. Ranked-choice voting does that. You don't have to vote for the candidate who has the best chance of beating the opposition. You rank your choices. First choice is the person who best represents you. After the votes are tallied, the candidate who gets >50% wins. If nobody achieves that, the candidate with the least votes is removed and the second choice of those who voted for them is used. This process continues until someone achieves the supermajority.
It has the advantage of doing away with the idea that you're wasting your vote by not voting for the candidate who has the best chance of prevailing against the opposition. If your candidate is removed, your second choice receives your vote. Your vote ALWAYS counts. A side benefit is that we no longer need runoff elections. Everyone's second (and third and fourth) choices are already taken into account.