The selling of NFA firearms are regulated at two levels. Dealers in NFA firearms are held to a slightly higher standard than a mere private owner transferring his firearms privately. It is true that both must register the sale of the firearm with the ATF, but the private owner is not subject to federal age requirements for the sale of a firearm. Rather, the private owner is only regulated at the state level.
In South Carolina, an 18 year old can buy firearms without violating state law and can legally possess the same. This creates a strange dilemma where 18, 19, and 20 year old residents can purchase and own firearms, such as pistols and NFA firearms, that dealers cannot legally sell to them.
How would someone in this age range acquire a NFA firearm or pistol? Buy it from a private owner. Of course, people in this age range may also serve as Trustees of a NFA Trust and the Trust itself may buy a NFA firearm directly from the dealer.
I strongly discourage appointing minors as trustees, but any person 18 years old or older can safely serve as a trustee. This allows for young persons to possess and use NFA Firearms before they reach 21. This is just one more minor benefit of using a trust to own NFA firearms.