Earlier quoted context omitted.
Depends on the state, but Federal law does require you to be 21 or older to purchase a handgun. Ammunition can be sold without any background checks or ID checks in some states.
21 to purchase from a retail, I believe it's still 18 to purchase from an individual, making it even more backwards.
Is that really so backwards? Seems to me that a personal sale is a more intimate exchange and it might be expected that the seller vets the buyer a bit more carefully than your average retail transaction, thus justifying the lower age restriction.