Vibe (the teen club for 14-17 yr olds) is very strict and will not allow the 18-yr-old except during scheduled Open House times. Likewise, "adult only" areas are strictly for those 18+ and would not allow the 15 yr old. However, if they wish to do things together there are still plenty of activities: movies, family pool area and AquaDuck, sports deck, towel-folding and other workshops/seminars, most lounges are open to all ages during the day, most of
Castaway Cay is open to all, most port excursions (CC or Nassau), anything on the Navigator under "family" or "all ages", etc. Your oldest might want to check into the 1820 Society, which is for that age range; go the first night because if nobody goes they won't plan any further meetings.
Enjoy your cruise!