Really ageism is an area of accepted double standards - denying Alzheimer's patients the right to vote is considered absurd yet even college graduate minors cannot vote despite being proven more capable than average. Or rare absurdities like a minor charged as an adult who managed to get acquitted in spite of the situation's seriousness isn't allowed to vote because of lacking the capacity.

Holding the elders to the same rationale is fair if not just - the situation isn't just for anyone affected but they aren't getting special treatment. To give an absurd example executing people for jaywalking is unjust but doing it to all regardless of station is fair.

I believe in keeping voting sancrosanct mind you but I can see a rhetorical and logical point to it.