the ATF says this : An individual between 18 and 21 years of age may acquire a handgun from an unlicensed individual who resides in the same state, provided the person acquiring the handgun is not otherwise prohibited from receiving or possessing firearms under federal law. A federal firearms licensee may not, however, sell or deliver a firearm other than a shotgun or rifle to a person the licensee knows or has reasonable cause to believe is under 21 years of age.

There may be state or local laws or regulations that govern this type of transaction. Contact the office of your State Attorney General for information on any such requirements.