Correct. Notice, however, that the age 'restriction' is vague? It doesn't require any proof although I suspect that a place like WalMart might ask for ID.
Rome
Minimum Age Restrictions
Federal law regulates the sale of ammunition to young people. It bars licensed manufacturers or dealers from selling long gun ammunition to anyone who they believe is under age 18. Also they may not sell handgun ammunition to anyone they reasonably believe is under age 21 (18 USC § 922(b)(1)).