You only need to be 18 to build on a Form 1. However, you will not be able to purchase the M92 from a dealer until you are already 21.

If you're willing to mark a new serial number (in addition to what's on there), there's no reason you couldn't submit a Form 1 right now, then go buy the M92 when you're 21, and mark down your own "maker" info along with the new serial number you wish to assign to it.

Or find someone to sell you an M92 in a private, FTF sale.

Or, if you can get the store to hold it for you, sure, you could submit a Form 1 for the serial number the store's holding for you.