Theoretically if you are buying a lower receiver and under 21 could you purchase someone’s “completed” rifle with the upper but afterwords tell them keep the upper and only pay the price of what the lower receiver if worth? If the lower is not a “virgin“ lower and is from a rifle could you purchase that at 18-20?
You joined on the 3rd of this month, and it seems from your previous posts on here you're looking for ways to try and get around not being old enough to buy an AR pistol. It doesn't look well when people come here looking for "loop holes".
There are some exemptions to the new restrictions though, here are some links that should help you find the answers you're looking for.
https://vaguntrader.com/forums/ubbthreads.php/topics/1656202#Post1656202https://vaguntrader.com/forums/ubbthreads.php/topics/1762534#Post1762534With that being said it's illegal to use a rifle lower to build a pistol without going through the motions to build an SBR (tax stamp, form 1).