There is some confusion on how old you have to be to buy a stripped or a complete AR15 lower receiver. Federal law states that you must be 21 or over to buy any firearm from a FFL. The exception is that you only have to be 18 or over to buy a long gun. A stripped or complete AR-15 lower receiver is just a receiver, it is not a long gun nor is it a pistol, hence the 21 or over requirement. Many people think that since complete lowers have a buttstock or a buffer tube already attached that it makes it a long gun. It does not. According to the ATF, it is not a long gun until an upper with a barrel of 16 inches or more is attached to the lower. Some people think the dealer can simply log the receiver in as a long gun. While it is true that some dealers do this, they are not supposed to. On the same hand, some people want to build a pistol out of a stripped lower and will try to have the dealer log it as a pistol. There again, some dealers will do this, but they are not supposed to.
The ATF has previously stated that if you buy a stripped lower and configure it as a pistol FIRST, you can then later configure it as a rifle. If you configure it as a rifle FIRST, then as a pistol, you have created a SBR (short barreled rifle), which falls under a whole additional set of regulations and requires a $200 tax stamp.
It is VERY important that you know or get advice from a dealer in your area that knows both the federal laws and whatever city and state laws you may have. The bottom line is that the shooting sports is a fun activity and has many great people participating in it, the last thing anyone needs to do is unknowingly break a law and put all that in jeopardy for the rest of your life.