Just found this (WARNING: page contains banner ads which include partial nudity and mild sexual content) whilst checking out referers to some of my web pages (someone who linked to one of my pages also linked to this article). Basically the US congress seems to have passed a regulation that adult content sites that feature content from or link to other such sites must ensure that models on those sites are of legal age.
Whilst there may be issues around the practicalities of this (e.g. what if a linked site features a new model without notifying sites that link to them, what constitutes a link (would an aggregated RSS feed count as a link) &c) the principle does seem reasonable. To take an analogy, if I own a bar and one of the staff knowingly serves an underage person with alcohol or sells a person who is of age with an alcolholic drink knowing that they intend to pass it to an underage person then I would be legally responsible and could be fined.