I'm not saying other age limits are reasonable, some might be others might not, some might be comparable to voting others are not. Criminal responsability for instance seems to be more about a basic understanding of right and wrong, I can see that having a lower age than voting.
Voting is not about asking a lot of people to come up with the best result, it's about giving the whole system the legitimacy to rule the people by force. It has to be as inclusive as possible and a certain amount of cluelessness or unengagement doesn't seem good enough a reason to deny the right the vote for me