Maybe first you need to actually talk about it, about why you aren't having it, why you aren't ready, why she isn't. Then explain that graphic discussions are frustrating and you want to avoid it in the future.
Is she teasing you? Trying to seduce you when you have agreed to wait? This is a game that some girls play, a way to feel powerful (and it is) and they sometimes find it funny to get you bothered when they know you have no intention of doing anything about it.
If this could be happening, you need to discuss that as well. She is awfully young (tho i have no problem with the differences in your ages as long as you wait until she is of the age of consent in your own state before anything physical happens). Some young girls discover the leverage of sex early and just find the whole thing a funny game. Do not let that get going with her.
If she really cares for and respects you, this conversation will go well and she'll knock it off. If she doesn't, she will continue to frustrate you with little innuendos and provocative behavior, at which point i would say you have a decision to make.
