Is there somewhere I can find this info at because this FFL needs the info, because apparently he doesn't know
http://www.atf.gov/publications/download/p/atf-p-5300-4.pdf
The problem is that you won't find a regulation or statute that specifically says the FFL can do something, such as receive a handgun from a person 18 years old, because 99% of regulations and statute define what is prohibited. Absent a prohibition on the action, the action is legal.
Best bet...call the local ATF field office and ask them if there is a statute or regulation that prohibits it, and if they say "yes", ask them which one.
Here is 18 USC 922 (b)(1), notice it only applies to selling or delivering, not receiving.
http://www.law.cornell.edu/uscode/text/18/922
(b) It shall be unlawful for any licensed importer, licensed manufacturer, licensed dealer, or licensed collector
to sell or deliver—
(1) any firearm or ammunition to any individual who the licensee knows or has reasonable cause to believe is less than eighteen years of age, and, if the firearm, or ammunition is other than a shotgun or rifle, or ammunition for a shotgun or rifle, to any individual who the licensee knows or has reasonable cause to believe is less than twenty-one years of age;
18 USC (j) is the prohibition against receiving a stolen firearm, notice the difference?:
(j) It shall be unlawful for any person to
receive, possess, conceal, store, barter, sell, or dispose of any stolen firearm or stolen ammunition, or pledge or accept as security for a loan any stolen firearm or stolen ammunition, which is moving as, which is a part of, which constitutes, or which has been shipped or transported in, interstate or foreign commerce, either before or after it was stolen, knowing or having reasonable cause to believe that the firearm or ammunition was stolen.