If the 18 yr old is a boy...YES! A higher risk of Illegal sex may be at hand after or during the prom! Yes it differs if the girl is 18(the chances are lower but still there). Most people who answer yes are girls, what girl doesn’t want to go to the prom with an older boy? And what older boy doesn’t want to get with a younger girl? It’s lose, lose situation and is it worth the risk. The prom is one of the biggest events to a high school student, but once college comes around you realize it was just another event to remember the worst or best day of HIGH School and nothing else.