Is a person allowed to buy a handgun in Arizona at age 20 through a private sale? This is not for me, I am well over 21, but I was asked, and don't have the answer.
Yes. There is no state law forbidding an adult from buying a handgun via a private party.
Here's the info. Under 21 cannot conceal carry but OC is good.
This is from the United States Code––––––––––––––––-
It shall be unlawful for a person to sell, deliver, or otherwise transfer to a person who the transferor knows or has reasonable cause to believe is a juvenile–– (A) a handgun; or (B) ammunition that is suitable for use only in a handgun. (5) For purposes of this subsection, the term ``juvenile'' means a person who is less than 18 years of age.
Here's the two statutes where it says it's OK in the Arizona Statutes––––––––––––-
4. "Firearm" means any loaded or unloaded handgun, pistol, revolver, rifle, shotgun or other weapon that will expel, is designed to expel or may readily be converted to expel a projectile by the action of an explosive. Firearm does not include a firearm in permanently inoperable condition.
Now combine that with this section from the AZ Statutes ––––––
A. Except as provided in subsection C of this section, a person who sells or gives to a minor, without written consent of the minor's parent or legal guardian, a firearm, ammunition or a toy pistol by which dangerous and explosive substances may be discharged is guilty of a class 6 felony.
B. Nothing in this section shall be construed to require reporting sales of firearms, nor shall registration of firearms or firearms sales be required.
C. The temporary transfer of firearms and ammunition by firearms safety instructors, hunter safety instructors, competition coaches or their assistants shall be allowed if the minor's parent or guardian has given consent for the minor to participate in activities such as firearms or hunting safety courses, firearms competition or training. With the consent of the minor's parent or guardian, the temporary transfer of firearms and ammunition by an adult accompanying minors engaged in hunting or formal or informal target shooting activities shall be allowed for those purposes.
From the ATF FAQs––––––––––––––––
(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]
(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)]
I hate to link the Brady Bunch website but ––––––––––––––––-
Is it illegal to sell handguns to anyone under 21 years of age? No
Arizona: State law restricts selling or giving firearms to juveniles under 18, except for supervised loans of firearms or for limited lawful activities (such as hunting). See also: Juvenile Possession.
Here's a news link saying that North Carolina tried to limit handgun ownership to 21+ and the Supreme Court found that unconstitutional since it conflicted with US law.––––––––––––––––
"But, a state law signed in April does allow private individuals to sell handguns to people 18 and older. The ruling brings South Carolina in line with federal law, which allows 18 year olds to own a handgun."
This web-page has the information from the two attached files that make up the pamphlet that the ATF distributed.––––––––––––––-
Just use Control+F (instead of searching the whole page) to find "unlawful for a person to sell" and this will get you to the same material that was included on the pamphlet.
Also on here if you dont want to look though the above.
Scroll down for the article titled "Legal Age to Purchase or Possess Firearms in Arizona…"
Thanks guys! I will relay the info!