Mimi - I think they'll fit right in. Yes, there will be more married couples without kids, but with RCI doing so much advertising on CBS during The Amazing Race & Survivor, they are attracting a younger, active crowd, IMO. Besides, there will be around 3400 passengers, so they are bound to find plenty of pretty young women to pursue! I'm sure they'll love all the activities the ship has to offer.
The kareoke bar and Promenade Cafe in the atrium are great places to hang out, besides all the other bars & disco. Don't worry - they'll have a great time!