This is covered by:
US TITLE 18 > PART I > CHAPTER 110 - Sexual Exploitation of Children
http://www4.law.cornell.edu/uscode/1..._I_20_110.html
Sites whose models/actors merely *look* like minors but are actually
adults must comply with USC 18 § 2257, which requires producers of
such sites to verify that their models/actors are over the age of 18,
and requires these producers to keep records of the performers name,
address, age, and every name, nickname or stage alias ever used.
Additionally, they must prominently display a disclaimer on the site
attesting to the fact that the performers are all legally adults.
TITLE 18 > PART I > CHAPTER 110 > § 2257 Record keeping requirements
http://www4.law.cornell.edu/uscode/1...7----000-.html