If the pictures have been sent there is the added charge of distributing child pornography.
If I would have known she was not 18, I wouldn't have talked to her at all, she was dishonest about her age.
Of course, one can hardly blame her for jumping at the chance.
More Until and unless you are contacted by the authorities I would just do nothing.
Finally, nothing good ever comes from sexting.
I met a girl on a sexual-encouter website 18+.
Every statement you make about this incident could constitute an admission that could be used against you in a potential prosecution.
More Technically, yes you could face prosecution.