Age requirement for...

[TURNKEY13]

Kina figured... Basically like possessing hi-cap mags.

[Mssr. Eleganté]

The operative word is probably purchase, not possess. Just like a handgun, an 18 year old can not purchase, but they can possess or own one.

Correct, and it only applies to acquiring through an FFL. If State law allows it then an 18 year old can purchase a pistol grip only "shotgun" or stripped lower from a private party when no FFL is involved in the transfer.

[Quiet]

Which brings up this question, if Quiet is correct, and im not disagreeing, if an 18 year old buys a stocked shotgun, then converts it to a cruiser style grip only, did he just commit a felony by possessing and illegal weapon due to his age? Or is the operative wording through an ffl?

No.

Per Federal laws...
Title 1 shotgun with the shoulder stock removed is still a Title 1 shotgun.
Because it started off as a shotgun, it retains it's classification as a shotgun. (this is the same for rifles & MGs).

[Futurecollector]

if you can buy an AR-15 at 18 you can buy a shotgun with a pistol grip at 18

You would be wrong,

as others have stated,

18 y/o shotguns/rifles,
21 y/o Handguns and others
---"Others" are Pistol grip shotguns as well as AR-15 Lower Receivers..

[Virus55]

if you can buy an AR-15 at 18 you can buy a shotgun with a pistol grip at 18

WRONG! You have to be 21 to buy a pistol grip shotgun

[Bigtwin]

As all of you have jumped on Reverend Clint, I am wiling to bet money he (like me) was thinking with a stock, not pistol grip "only" after reading Bjkerns post!