In the state of Montana, under state law, you have to be 14 years or older to legally possess a rifle. Obviously it would have to be a gift or possibly a purchase from a private individual since the Gun Control Act (GCA) states that you have to be 18 years or older to purchase a rifle from or through an FFL. As the letter in the link below states, the "NFA is silent on age restrictions" so would it be legal for a 14 year old named as a trustee in a gun trust to be in possession of a suppressor on a rifle without the supervision of an adult trustee also named in that trust? This question is not academic. I set up a trust for a friend who has two boys 14 and 15. They currently have a number of suppressors that I transferred to them and they are all avid hunters. The dad wants to know if his boys can use the suppressors without his supervision. Good question.
https://princelaw.files.wordpress.com/2011/09/18_years_old_to_own_nfa.jpg
https://princelaw.files.wordpress.com/2011/09/18_years_old_to_own_nfa.jpg