You can volunteer for the military at 18 and should be able to vote if you do so, except you should not be able to be drafted until you are 21. Also, raise the voting age to 21 for non-military.
How about we make all "adult" things happen at the same age?
Drinking, signing contracts, joining the military, being drafted, being tried as an adult, paying for a hooker, voting....
They should all be the same age, you are either an adult or you are not, this stupid piecemeal is for the birds.
Oh, and that age should be 18