It isn't. If it left the factory classified as a handgun (ie, with no stock), then the purchaser must be 21. Lower receivers are restricted to buyers 21 and over due to a recent ATF policy change.I'm not sure if this is 100% accurate.
However, a normal AR-15 is a rifle, which means that it can be purchased by someone 18 or older. Some retailers have their own policies restricting sales of "assault weapons" to purchasers over 21, but federal law just calls them all rifles.