its not the mental ability that defines when you can vote. You have to be old enough to be liable for your decision in who to vote for. And you have to be capable to handle the repercussions as well.
it is the right and duty of a CITIZEN to vote, and if i remember right, children are not considered citizens cause they are still being taken care of and is responsible to the family unit, not the state, at least in social constructs that practice democracy.