There is no law that says that.
18 for long guns; 21 for hand guns
Hand gun - 21; long gun - 18
21 for hand gun 18 for long gun
Federal law specifies 21 or older for a handgun and 18 or older for a rifle or shotgun.
technically you could be 10 as long as you have an adult (over 18) supervizing so there is no need to be 21.
21.
21
21
21
18 for long gun, 21 for hand gun
If you are not a felon, then the legal age is 21.
18 for rifles, 21 for handguns