How Old Do I Have To Be To Buy An AR-15 Receiver?
According to the Bureau of Alcohol, Tobacco, Firearms and Explosives, you MUST be 21 years old to buy a stripped or complete lower receiver from a licensed dealer (FFL). This is due to their classification of a receiver not being a long gun. It DOSE NOT matter if it has a stock attached or not, you still must be 21 years old to buy a lower receiver. If you are 18 years of age, you can buy one from an individual in your state of residence in a face to face transaction, or have one gifted to you from your parents/grandparents.
You must also purchase/transfer it in YOUR state of residence.
According to the ATF, once you purchase a stripped or complete lower receiver, if you remove the stock and attach an upper with a barrel length of less than 16 inches, you can then build it into a rifle then later you can turn it back into a pistol. If you attach an upper LONGER than 16 inches first, IT IS FOREVER A LONG GUN, if you later build it into a pistol, you will need to go through the process to build a short barreled rifle.
So, when I buy a stripped or complete AR-15 lower, I configure it into a pistol FIRST, that gives me the flexibility down the road to switch back and forth between pistol and long gun.