I can't comment on what happens in MA from the local perspective, but ATF does not, as a general rule, inspect the end-users. If you are a dealer, you *may* get a once a year compliance inspection. Collectors do not get compliance inspections. If ATF comes a calling, you have other problems.
For the initial poster, I'd suggest you start buying your class III items NOW. The prices are going to continue to escalate, but more importantly, the supply is drying up. I'm not sure where the age 21 thing is listed in the rule book, but I do remember hearing that in the past. You could, however, purchase the items NOW and have the gun delivered to the dealer of your choice and have him sit on them until you are 21. Of course you'd have to trust him. And you'd have to have the cash. Or have a family member purchase the guns for you and transfer them to you when you're 21. It wouldn't be a strawman, since he/she would actually buy the guns for him/herself. Of course they could not be "loaned" to you -- the person would have to be with you when you when shooting.
Shoot, I wish I started collecting when I was 18 (or even 21).
danbrew :->