The National Firearms Act (NFA) contains no minimum age in regard to the possession of NFA firearms. Regardless, federal firearms law prohibits a licensed importer, manufacturer, dealer or collector (i.e., licensees) from selling or delivering any firearm or ammunition to any individual that the federal licensee knows or has reasonable cause to believe is less than 18 years of age. Federal firearms law further provides that a licensee shall not sell or deliver any firearm or ammunition that is other than a shotgun or rifle or ammunition for a shotgun or rifle to any individual that the federal licensee knows or has reasonable cause to believe is less than 21 years of age.

Quoted:
Good to go with existing items.  New form 4s will be denied unless the under 21 trustee is removed.

Quoted:
The real question is, is your 18-yr old son trustworthy (not a pun…well, yea it is) enough to not jeopardize the trust itself by engaging in activities or actions he may consider normal at his age (e.g. smoking pot) or an oversight/accident on his part (e.g. traffic accident).

While you may think he’s not doing anything wrong, you most likely will never know if he is/n’t.

The better course of action from a legal standpoint is to make him a successor trustee.