Thanks for the quick reply, Chanti.
Yes. Characters must be physically mature. By this, we mean girls MUST be menstruating and be mature enough to pass for 15 in appearance not to mention have physically been alive for at least 15 years. Sure, some girls have smaller chests as not all physically mature girls have breasts that are Cs or DDs, etc. And they should be old enough to grow pubic hair--extenuating circumstances aside, of course (not counting medical reasons or shaving, etc). Same goes for guys. And the same goes for anyone or even anything...since we've had people try and skirt this with beasts, animals, aliens, you name it. It's just easier to keep it the same across the board as it offers less confusion for all since it's less to keep track of.
Now, some creative leeway can be allowed depending on circumstances. Example, you are certainly allowed to use minors as side characters to help strengthen your story, to give it depth, etc. The big thing here is that the minor is NOT a main character, they are NOT to engage in any kind of sexual act AT ALL (meaning, no sexual intercourse, abuse, torture, rape, objectification, etc etc etc the list goes on...just NO to anything regarding sexual). Another example, you are certainly allowed to have your main character have a past in which they might have suffered some kind of sexual something as a minor. Now, you might not write out the details of it, but it can be alluded to and it can be used as a literary device in how your character might act, think, etc as you write them to help strengthen your story and add to overall character development.
As for the standard typical......the character looks 8, but is actually 3000 years old because they were turned into a vampire at 8 and since they've been alive for 3000 years they have the mind of an ancient, blah blah blah.........sorry, this is just a no-go. And anything like this? Yeah, you'll be denied. Mind might be fine, but the body is NOT mature and hardly passes as a 15 year old. Now, if you made the character 15 and did the same deal? You'd be fine. But do not come to me or anyone else on staff requesting such a thing. It will just get rejected.
Hope this answers your question and any others that might be forming in your head regarding this rather touchy topic. I also hope it provides some insight to anyone else out there who happens to read this particular question. So, thanks for asking. : )