Europe (and the Mediterranean) are growing in popularity. And fast. Try and book a cruise with Royal Caribbean, Holland America, Princess, Celebrity, or Norwegian to the Caribbean over the summer. Can't? Well, the majority of their fleets were repositioned to Europe. Carnival is starting to accommodate this trend, but they still seem to favor the Caribbean (and rightfully so, since everyone else left).
I would estimate that your cruise will have a significant number of teens onboard. However, the fact that you are cruising Celebrity does limit that number. I know this isn't "exact" but predicting demand is rarely an exact art. People studying economics have a hard enough time doing it.
Consider the following (price [directly related to price: length of cruise, cruise line, and sometimes itinerary], itinerary (when not related to price), ship (age, amenities, teen amenities, teen friendliness), and embarkation port (how easy is it to get to where you leave from?))