Can a guy still get arrested if he was with a minor before but now the girl is over 18?

The manor in which the question is asked implies sexual contact. This being the case, it is important that you understand that many states have repealed their statutes of limitations regarding sexual abuse of minors. That being said, anyone who engages in sexual contact with a minor can be arrested in many jurisdictions 100 years from when it occurred or later if he lives that long. It does not depend at all on the current or future age of the then minor.