>Old enough to defend their country
>but not old enough to defend their family.
Ol guy observation. My uncle was a top turret gunner of a B17 in WW2. He was third in command of the aircraft. He was responsible for the engines and armament prior to take off. He was mature.
Today's 28 year olds, living in mom's basement, and playing video games are not mature, not to mention the 18 yr old beta's in HS.
Simple solution to the 21 year age limit (already in place for hanguns):
If you enlist in the military and pass boot camp you get a waiver/license to own handguns/long guns at 18. Military folk mature quickly.