Allow for any and ALL Military personnel to carry firearms (In accordance with respective states' laws) regardless of their age. If you're old enough at 18 to be drafted to fight for your country and carry an M16 or automatic weapon, then you are old enough to carry any weapon here at home. The Federal Government cannot force the states to allow all 18 year-old's to carry because it is not their jurisdiction BUT they CAN allow Military members to carry because the Military IS their jurisdiction.
This law would ONLY apply to members of the Military whom have graduated high school and have completed Basic Training. They would still have to apply for state permits (Where needed), complete qualifications and follow restriction laws where applicable (Open Carry only, et cetera).