I see. You're not in any trouble now, as there was no indication that she was underage (in fact, the contrary). Now if you knowingly send nude photos to a minor, that is an offense. So you should say that you absolutely will not. My guess is that this is a midpoint in the scam, in that once you do send photos to her "knowing" that she's underage, that's when you'll be blackmailed with money. Now I bet that this "girl" doesn't even exist. That is, it's a scam. It's probably some middle aged man that's using this, and the fear of it getting out.
Again, no crime has been committed. It requires knowingly sending lewd photos to a minor. Say that you're not going to, because at that point it would be a crime. Say that you're willing to take this to the police if she does, as it's a classic trap and blackmail situation. I doubt that you'll hear anything more about it.
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for my answer unless and until you rate it a 3, 4, 5 (good or better). Thank you, ***** ***** luck to you!