My cousin says there should be a maximum voting age. She says if people can't vote for the first 18 years of their life, why should they be able to vote for the last 18 years (e.g. if retired or older than 65)?

I disagree, of course. I don't talk to this cousin anymore. Personally I think the very elderly contain the wisdom for future generations, and make the best decisions because of all that wisdom, so they should really be in charge of everything.