One common question and often misunderstood is the age requirement to buy either a stripped ar15 lower receiver or a complete ar-15 lower receiver (one with a stock attached). To buy any title one firearm in the US, you must be 21 years old. The only exception to that is a shot gun/long gun. In the eyes of the ATF, it is a receiver. It is NEITHER a HANDGUN nor a LONG GUN. It DOES NOT matter that it has a stock on it. According to the ATF, if you are over 21 years of age and buy a complete lower, you can remove the stock assembly and install an upper that has a barrel length of less than 16 inches (thus building a pistol). You can later take off the less than 16 inch upper, reinstall the stock assembly and attach an upper with a barrel length OVER 16 inches (thus building a long gun). After starting the initial build as a HANDGUN, you CAN go back and forth between handgun or long gun.
If you buy a complete lower and initially attach an upper over 16 inches, IT IS FOREVER a LONG GUN. Without a $200.00 tax stamp and some paper work, it will always be a long gun.
If you buy a stripped ar15 lower receiver, you can initially install an upper with a barrel less than 16 inches (building a pistol) WITH NO STOCK ASSEMBLY ATTACHED. You can later configure it into a long gun, following the steps above.
If you are over 18 years old, you can (depending on state laws) buy a stripped or complete lower in a face to face transaction from another citizen of the same state that you are in. To purchase/transfer from an FFL, it MUST take place in your state of residence and you MUST be 21.
Some people make the mistake thinking that they can “just have their dealer designate it as a long gun on the 4473”, while you may find a dealer that DOES NOT KNOW THE LAW and will do that, in doing so, they may be putting their FFL in jeopardy.
A stripped or complete ar15 lower receiver is NOTHING MORE THAN A RECEIVER and therefore MUST be recorded as either “Other” or “Frame/Receiver” on the 4473.