quick google search (dunno how accurate it is):
Does federal law place any age restriction on ammunition sellers and purchasers?
Under federal law, it is illegal to sell (1) long gun ammunition to anyone under age 18 and (2) handgun ammunition to anyone under age 21 (18 USC 922(b)(1), 27 CFR 478. 99(b)). According to BATF, a licensee may sell interchangeable ammunition such as a . 22 cal. rimfire to a person less than age 21 (but age 18 or older) if the dealer is satisfied that the ammunition is for use in a rifle. If the ammunition is intended for use in a handgun, the 21-year-old minimum age requirement applies.
guess he was right, though I've never tried purchasing ammo when I was younger...
18 USC 922:
27 CFR 478.99