Since when you ask?
Since Western civilization as we know it decided that an adult is someone over the age of 18 (or 21) you twit.
So long as the girl has ID proving she's of age, who fucking cares how young she looks?
You obviously need some marketing lessons. Let me give you a quick one. The younger the looks, the more attention the site gets. More attention=more sales.
Unless you're just a bible thumper
