we've taken our teenagers the last three years and each year our son as kicked it up a notch for formal nights. Last year (he was 15) and wore dress pants and white formal shirt and dad's tie one night and attempted to rent a tux the next formal night. The teens on the Grand Princess were definetely dressing up. This year he has us purchase a suit and dress shoes and wore those on formal night.
The teens on the RCC (Rhapsody) didn't quite dress up as much as the teens on Grand Princess but he felt handsome. Don't prejudge our teens based on what we are used to them wearing - they also love to be a notch above the daily usual when traveling on a cruise.