People at 18 can make/register their own NFA items. People at 18 can buy/own NFA firearms if they come from a non licensed person.
People have to be 21 to buy NFA from a dealer.
Same applies to title one handguns and pistol gripped shotguns. 18 to make or own, 21 to buy from licensed dealer.
I can explain it to you over and over, but I can never learn it for you.