Amazingly, as a minor myself, I had a very similar idea. Mine had better symmetry, this one would be easier to implement.
This idea, is simple- 14 & 15 year olds get 1/4 of a vote, and 16 & 17 year olds get 1/2. This way, the kiddos get a chance to learn the ropes without fucking things up too badly, or skewing the vote.
My version of a similar idea was targetted for the federal level. You treat the total population of minors, divided by 3.5 in the US as a single state (voting minors mind you). Give them electoral votes, representatives, and two Senators.
I had a ten page plan written out on it, and being in fifth grade, I think that shows an interesting amount of consideration.