At least 18 for rifles and shotguns.
18 for long guns, 21 for hand guns.
Federal law (and most states) require you to be 18 to PURCHASE a shotgun from a dealer. You may OWN a shotgun at any age, but if under 18, will need the consent of your parents.
at least 18 and over, and it must be licensced. It depends where you live.
In the U.S. 18 for rifles and shotguns, 21 for handguns.