There’s a reason why under 18’s are not allowed to vote and it’s the same reason why they’re not allowed to drink either. Voting is a responsibility. To allow children who haven’t fully matured into self aware responsible adults (and one could argue that most 18 year olds don’t even manage that) a say on who runs the country would be irresponsible.
As for extending the proxy system to allow guardians to vote on behalf of others that would be a misappropriation of the system. It would just be handing extra votes to individuals with no guarantee that those votes are being made according to the opinions of the original owner of the vote.
I for one wouldn’t agree to handing my vote over to my parents as I know that my vote is likely different to theirs.
