18 USC 922.
http://www4.law.cornell.edu/uscode/1...2----000-.html
(b) It shall be unlawful for any licensed importer, licensed manufacturer, licensed dealer, or licensed collector to sell or deliver—
(1) any firearm or ammunition to any individual who the licensee knows or has reasonable cause to believe is less than eighteen years of age, and, if the firearm, or ammunition is other than a shotgun or rifle, or ammunition for a shotgun or rifle, to any individual who the licensee knows or has reasonable cause to believe is less than twenty-one years of age;
BATFE's interpretation:
(F7) May a licensee sell interchangeable
ammunition such as
.22 cal. rimfire to a person less
than 21 years old?
Yes, provided the buyer is 18 years
of age or older, and the dealer is satisfied
that it is for use in a rifle. If the
ammunition is intended for use in a
handgun, the 21 year old minimum
age requirement is applicable.