I'm not condoning the activities of the person(s) who posted that ad, assuming they actually are into child exploitation, but I thought I should answer Smash.Hit's question.
Legal age of consent differs from country to country, and in the US from state to state. The universal minimum age of consent for work in adult entertainment is in fact 18. Thus while one might be able to legally consent to intercourse at a younger age (for example, here in Canada it's only 16), they cannot take nude photos or video of themselves having sex until they're at least 18.
Whether or not you agree with the law, those are the legalities of consent (as far as I know) here in Canada.
If what I've read on this thread is true, then it sounds like the girl in this ad is actually younger than 18. Possibly old enough to legally fuck, but certainly not old enough to legally do porn. Personally, I wouldn't risk it.