Molest is actually an inappropriate term. “Molest” means to bother, harass, annoy–think of the signs you see at the zoo. The 18 year old would be a “sex offender.”
That said, he is not “sex offender” as the 18 year old participated in consensual sex with a 17 year old girl. This needs to be taken in context. If he was an 18 year old impregnating a 12 year old child, this would almost certainly be sex offense. Teens of 17 and 18 have little difference in emotional and mental development. There is nothing to suggest that Bristol Palin was coerced, manipulated and physically forced to have sex.
If it was a 35 year old and a 17 year old, once again, the 35 year old would be a sex offender. There are some very big differences in development.
Should we label 6 and 5 year olds playing “doctor” and looking at each other’s body’s parts, as sex offenders and victims? Let us hope not. However, if it a 14 year old and 6 year old, the story looks a little different.