Canada

There's absolutely no argument denying youth the right to vote that wasn't equally invalid when it was used against other groups. There are uncaring, naive, uninformed, and stupid people in every group, but denying the whole group is wrong. It's also going to be inconsequential -teenagers make up a tiny fraction of the population. The main reason to do it is that voting young predicts lifetime engagement in the voting process - so limiting it is a bad idea for that reason alone.

I'll go even further and say that anyone with the mental capacity to be able to follow the rules and instructions, maintain decorum in a polling station, and properly fill out a valid ballot should be allowed a voice in elections. That's the same criteria we use for legal adults.