It could be diseminating indecent material to a minor and violate federal and New York state law.
I am a former federal and State prosecutor and now handle criminal defense and personal injury/civil rights cases. Feel free to check out my web site and contact me at (212) 577-9797 or via email at Eric@RothsteinLawNY.com. I was named to the Super Lawyers list as one of the top attorneys in New York for 2012. No more than 5 percent of the lawyers in the state are selected by Super Lawyers. The above answer is for informational purposes only and not meant as legal advice.
Yes, there could be liability for engaging in this type of activity.
An 18 year old should know better, and a 15 year old is protected by the law because they aren't old enough to make their own decisions yet.
I recommend you suggest these friends stop talking about sex - and definately DO NOT think about sexting! Then their would be serious child pornography complications.
This post is provided for general informational purposes only and is not intended to be legal advice specific to you. This general information is not a substitute for the advice of an attorney in your jurisdiction. The attorney client relationship is not established by this post.