I can't imagine living in NY. I've only lived in conservative states with few gun restrictions. I'd say call New Frontier, tell them you're under 21, you live in NY, and you want to order a complete rifle lower from them. They should know if they can send it to your FFL or not. Either way, once it gets to the FFL, they'll call in a background check when you go to pick it up. It doesn't take long, but they're not going to hand you an illegal gun. There's a process that needs to be completed before you can take ownership. It doesn't take long. I wouldn't worry about it. First step is call NFA and tell them your situation.