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.