You need to be 21 to purchase a handgun from a dealer.
This was not a handgun.
Question still stands: how the fuck did he get a semi-automatic gun if he wasn’t even able to get a handgun?
Where are you from, exactly?
There’s no classes of licenses like that in the US. If you are 18 and meet the minimal legal requirements, you can buy a long gun of any type in most states. (Some states are trying to move that age to 21.) That means a single shot, break action, lever action, bolt action, pump, or yes, semi-automatic. Once you hit 21, you can buy handguns. Again: that includes break action, revolvers, and normal semi-automatics.
The only real restriction in all of this is machine guns; to get those, you need to come up with the $20,000+ that a legal one will cost, and file a transfer application with the BATF, pay a $250 fee, and wait to see if your application is approved or denied. There are some states that prevent individual ownership of machine guns entirely.