He has had some pretty bad senior moments, the worst one I can recall is the “Hey Jackie” one. In general he does talk pretty slow and just appears confused sometimes. He talks like he is really having to think about what he is saying, and that’s not just him, he didn’t use to be like that.
Voting isn’t actually a constitutional right like owning firearms is. There are protections about equality when it comes to voting, but not much about voting itself. States are generally given the right to decide who can vote and how they vote.