I already addressed that in reply number 17Re-read post #6
What the lower CAN be built into has ZERO effect on what it IS now.
That same lower can be built into a pistol, or with the knowledge and parts a machine gun, however, that has NOTHING to do with what it is NOW.
You would be well served ignore the "it can be a pistol" thinking and look at what it is NOW, or rather what it is not.
It is not a longgun because it does not meet the definition of a longgun.
It is not a pistol because it does not meet the definition of a pistol.
It is not a shotgun because it does not meet the definition of a shotgun.
On the Form 4473 it is Identified as an "other" as it does not meet any other definition. And as such, the licensee can NOT sell a firearm OTHER than a rifle or shotgun to anybody under the age of 21
If the lower is technically a "firearm" and you have to be 21 to buy a "firearm" in the legal definition, then it would make a little more sense. I'm currently googling what the legal age requirement to buy a firearm is, not something easy to find. My point is a stripped lower is not a pistol, therefore I'm not sure why it should have to be treated as a pistol just because it can be turned into one.
If it does not fall under any definition for rifle, pistol, shotgun ect. it would be an AOW I assume? I can't find an age limit on owning an AOW, but with the assumption than an AOW falls under the NFA, I should still be legally able to buy a lower at 18, because it is not a pistol, and not currently requiring NFA registration. This is my interpretation.
Article on being able to own an NFA at 18+
https://www.guntrustlawyer.com/2015/05/can-an-18-year-old-build-an-sbr-sbs-or-other-nfa-firearm.html
Also it says you must be 21 to buy an NFA item from an FFL, but 18 to build one. Since it is not an NFA item, or a pistol, I see no 21+ requirement applicable.
I want to say again that the gun shop is awesome, they're being safe which is completely understandable. I will be going back when I'm 21, buying, and then having the items transferred to an FFL in my state.