The restrictions on handgun sales to those under 21 apply only to licensed persons (FFL's).
From the ATF FAQ
http://www.atf.treas.gov/firearms/faq/faq2.htm#b2B. UNLICENSED PERSONS
(B1) To whom may an unlicensed person transfer firearms under the GCA?
A person may sell a firearm to an unlicensed resident of his State, if he does not know or have reasonable cause to believe the person is prohibited from receiving or possessing firearms under Federal law. A person may loan or rent a firearm to a resident of any State for temporary use for lawful sporting purposes, if he does not know or have reasonable cause to believe the person is prohibited from receiving or possessing firearms under Federal law. A person may sell or transfer a firearm to a licensee in any State. However, a firearm other than a curio or relic may not be transferred interstate to a licensed collector.
[18 U.S.C. 922(a)(3) and (5), 922(d), 27 CFR 478.29 and 478.30]
(B2) From whom may an unlicensed person acquire a firearm under the GCA?
A person may only acquire a firearm within the person’s own State, except that he or she may purchase or otherwise acquire a rifle or shotgun, in person, at a licensee's premises in any State, provided the sale complies with State laws applicable in the State of sale and the State where the purchaser resides. A person may borrow or rent a firearm in any State for temporary use for lawful sporting purposes.
[18 U.S.C. 922(a)(3) and (5), 922(b)(3), 27 CFR 478.29 and 478.30]
B5) Are there certain persons who cannot legally receive or possess firearms and/or ammunition?
Yes, a person who –
(1) Has been convicted in any court of a crime punishable by imprisonment for a term exceeding 1 year;
(2) Is a fugitive from justice;
(3) Is an unlawful user of or addicted to any controlled substance;
(4) Has been adjudicated as a mental defective or has been committed to a mental institution;
(5) Is an alien illegally or unlawfully in the United States or an alien admitted to the United States under a nonimmigrant visa;
(6) Has been discharged from the Armed Forces under dishonorable conditions;
(7) Having been a citizen of the United States, has renounced his or her citizenship;
(8) Is subject to a court order that restrains the person from harassing, stalking, or threatening an intimate partner or child of such intimate partner; or
(9) Has been convicted of a misdemeanor crime of domestic violence
(10) Cannot lawfully receive, possess, ship, or transport a firearm.
A person who is under indictment or information for a crime punishable by imprisonment for a term exceeding 1 year cannot lawfully receive a firearm.
Such person may continue to lawfully possess firearms obtained prior to the indictment or information.
(B14) May a parent or guardian purchase firearms or ammunition as a gift for a juvenile (less than 18 years of age)?
Yes. However, possession of handguns by juveniles (less than 18 years of age) is generally unlawful. Juveniles generally may only receive and possess handguns with the written permission of a parent or guardian for limited purposes, e.g., employment, ranching, farming, target practice or hunting.
[18 U.S.C. 922(x)]
