This is an awfully wrong approach. As with things like Alcohol, if you teach your children how to operate and act responsibly, they get into less trouble with it as an adult.
If you stilt their growth and don't allow them to touch a gun until they're 18, they won't have developed enough to handle it themselves without supervision.
Starting before 21 legally != Starting before 21 illegally
You'd be surprised how much of a difference it makes when you remove the rebelliousness and allure of doing something illegally. You see this time and time again with parents sheltering their children from certain activities or harsh realities.
If you stilt their growth and don't allow them to touch a gun until they're 18, they won't have developed enough to handle it themselves without supervision.