21 years old and go through the nfa registration process for the firearm you want to have become a sbr. Once you get your 200$ tax stamp then you can have your receiver engraved with the required info and then purchase your short barrel and assemble an sbr.
The route i took was:
Purchased a regular stripped lower and got it engraved with the name of my trust.
Then assembled it with LPK and pistol buffer tube.
Purchased a 10.5" upper receiver group and put it on my lower.
Brought it to local metro and got it registered as AR15 pistol.
Submitted the Form 1 with my trust and $200 check.
For the time being while waiting for the stamp to come back, i could shoot it
When stamp comes back, swap the pistol buffer tube with rifle buffer tube with stock.
Why i chose this route? Cos it was so much cheaper than buying an already built SBR from manufacturer. I got the joy of building the lower, choosing the parts and i could shoot it. Rather than it being kept at my LGS during the whole 6+ months.
Most importantly i confirmed the legality of the steps i was going to take, by calling ATF and asking opinions from my LGS and someone in this forum. Thank's to that someone
Here is my AR15 Pistol: