Not calling you out on it or anything, VUPDBlue.....
But I think many people assume its 21 for NFA, since, well, doesn't that sound reasonable?
I have read the regs somewhat extensively, and don't recall an age mentioned. Maybe the Feds thought that the states would take care of that? Or there just wasn't too many teenagers buying machineguns in the 1930s?
Regardless, whether it was an administrative oversight (possible, I admit) or not, I can absolutely assure you my buddy aquired a full-auto AR-15 conversion 20+ years ago at the age of 19.
Then, as now, even in "gun-friendly" Arizona, the Phoenix PD, nor the Sheriff's Office, would "sign off". I confess, we haven't tried pushing them on it lately. We do have the County Attorney willing to do so, however, and we have been cruising along in that mode for quite some time.