Voting is already discriminatory based on age. We don't let 5 year olds vote, drive, or drink; why shouldn't we impose the same restrictions on say, a senile 95 year old with the mental age of 5 year old? I know why, implementing those restrictions would be far more difficult, but still, it's incorrect to say you should never discriminate based on age.
Ironically, I actually know a radical anti-ageism activist who thinks 5 year olds (and 0 year olds for that matter) should have all the same legal rights as adults.