Does anyone remember a movie from late 70's/early 80's that involved a rock star (maybe just a local band guy who falls for a young girl who is passing herself off as older. I thought it was something like Jessie's Girl. Can't find it.

Might be "Great balls of fire" I know in the movie he gets into trouble for the girl being too young

I know that movie. I think you mean Sooner or Later with Rex Smith. I had a poster of him on my wall!