We are often asked if you have to be 21 to build an SBR or Suppressor. If you are manufacturing a firearm under the NFA using an ATF Form 5320.1 you only need to be 18 under federal law. Likewise, if you are purchasing a firearm using an ATF Form 4 from an individual, trust, or non FFL you only need to be 18 years old. If you are buying one from an FFL, you must be 21 years old under federal law. Your state may impose stricter requirements.
Below is an excerpt from the attached letter from ATF explaining the age requirement.
This is in response to your letter of November 16, 2006, to the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) advising that you are 18 years of age, the owner of a AR-15 type receiver, and wish to register it has a short-barreled rifle using an ATF 1 application.You ask whether ATF would approve the application submitted under these circumstances.
You state that your understanding of the Gun Control Act prohibits the transfer of Short Barreled Rifles, but that you cannot find any language in the GCA nor the NFA dealing with the already possessed rifle receiver. You are correct. The GCA does establish age requirements for the sale of firearms by licensees and the NFA is silent on age restrictions.
Accordingly, since the GCA does allow possession of rifles by persons under the age of 21, you would not be prohibited from applying on a Form 1 to make a short-barreled rifle.
While an application by an individual would need photographs and a certification from your chief of police or sheriff, an application using a Gun Trust would not.