What Ken meant to say is that you can't carry on your body in public until you are 21.You'd think gun toting Fudd Kemp would think to consider a person's own body their property but I guess that starts at 21.
Even then there are exceptions for hunting, fishing, competitions.
There are all sorts of things that you can't do with your body because they are age restricted, screw, drink, smoke, dip, drive, vote.