Pretty sure that it is the "Traci Lords" law. She started making porn (I think at 15), but had fake IDs saying she was over 18. She won starlet of the year when she was 17. After she legally turned 18, an anonymous person disclosed her actual age, just after she had completed producing and starring in her first legal pron film. She was hugely popular and suddenly all of her films except for the new one were completely illegal to possess or view....suspicious, eh?
Anyway, they tried to prosecute all these directors and producers who had unkowingly filmed a minor, but had all thought she was of legal age. So this law was passed to ensure that they were checking ID and verifying the ages of the actors and actresses to prevent something like that happening again.
I had only ever read the blurb at the beginning of the movies, that is the first time I had seen the whole thing. Very interesting to see. Thanks!