I don't think it's unreasonable for voters over a certain age to have restrictions put on their voting. My grandfather died at 94 and for the last ten years of his life he was extremely misinformed, and I don't mean just because he voted in a way that I wouldn't, or got his news elsewhere. He lost the ability to properly understand news from the television/newspaper; he would frequently ask my family if we had heard of some story and then provide a wildly inaccurate summary of it, only loosely related. This was mostly due to the progression of Alzheimer's. He was still able to vote during this time and I think even participated in the 2016 Republican primaries (at that point he was having trouble remembering even the most basic of things).

That said, removing people's right to vote is not only difficult, but can cause more problems than it solves. Implementing voting tests can codify racism/sexism/etc. Medical tests will just be gamed. And good luck ever passing these restrictions anyway, since old people are the most consistent voters out there.