Refer to question #18 on the ATF form 4473 that you must fill out. The explanation on the back of the form clearly states 21 for stripped receivers.
http://www.atf.gov/forms/download/atf-f-4473-1.pdf
Question 18. T)'pe of Fireal'm(s): Check all boxes that apply. "Other"
refers to frames, receivers and other firearms that arc not either handguns or
long guns (rifles or shotguns), such as fireanns having a pistol grip that expel
a shOlgun shell, or National Firearms Act (NFA) firearms.
If a frame or receiver can only be made into a long gun (rifle or shotgun), it is
still a frame or receiver not a handgun or long gun. However, they still arc
"firearms" by definition. and subject to the same GCA limitations as any other
firearms. See Section 92 I(a)(3)(b). 18 U.S.C. Section 922(b)(1) makes it
unlawful for a licensee to sell any firearm other than a shotgun or rifle to any
person undcr the age of 21. Since a frame or receiver for a firearm. to include
one rhat can only be made into a long gun, is a "firearm other than a shotgun
or riflc." it cannot be transferred to anyone under the age of 21, Also, note
that multiple sales forms arc not required for frames or receivers of any
firearms, or pistol grip shotguns, since they are not "pistols or revolvers"
under Section 923(g)(3)(a).