I think State law is the applicable law here. In Texas, for example, it's illegal for someone under 18 to possess a pistol, except under supervision by a parent or someone designated by the parent in writing. I think there are other exceptions, as well, but they tend to be for various organizations. So, you could buy the pistol and give it to someone between 18 and 21 with no restrictions, or to someone under 18 as long as the parents approve.