I agree with Dork and Ben, if you are going to train an under 21 person fight and kill in the service, then that person more than deserves the rights of a 21 year old person. They will see and know much more than the local HS quarterback about how to act responsibly. Of course they may still try to date the head cheerleader, but at least they will have enough money to pay for the dinner and movie instead of going dutch.
I do not know the laws in other States but in Texas youngsters have to take some classes before getting their hunting license and have to hunt with an adult. When I was in 5th grade, that would make me around 11, I had my own 20 ga Mossberg bolt action to hunt dove and quail and a Model 700 270 to hunt deer with. Wish I still had the 270, it would be worth a small fortune to me.