Also, is it possible I could actually be prosecuted for something like this?
Absolutely not. 18 and over is legal consent in any state or country in the world. In some places it could be lower.
If she says, "19" before anything gets going, you are safe. If the person later changes the age after the fact, then they are breaking the law. They are the criminal, not you. Any threats of either posting material, whether they actually do it or not is a crime. Posting material without your consent is also a crime in most places. Extortion for "more pictures or videos" or money is a federal offense.
Please do go to the link Big Al suggested. http://blackmailscams.com
We can investigate using the EXACT details you have. If we cannot determine 100% that the person is a scammer, we will hold them hidden from view in case another person reports that they did threaten or attempt to extort money.