Count me with Tim and TAB on this, while the average 18-21 year old is not mature enough, There may currently be,(depending on state ) and should be, an exemption for military personnel, (You can enlist in the Guard at 16 )
As a side note, TAB can check if this is still the case, but in 2000 when I lived in Ca. some of the more remote counties issued an open carry permit to 16 year olds for Ranch work.