Hey Anna. The cruise travel agent is probably referring about the new "Z-class" ships. The first one, ZUIDERDAM, came out in December of last year and the second, OOSTERDAM, just debuted last month. HAL claims that this class of ships is targeting towards the younger age. In my opinion, these ships are way of the "old" HAL image.
The interiors are "exotic". Gone are the welcoming, elegant looking colors and these "Z-class" have bright, wild colors. These ships are big, weighing in about 85,000 grt and are based off the popular Carnival Spirit-class. First time in a newbuild for HAL is a specially designed center for teens and kids. There are two rooms in this center. One is for the small cruisers and the other is for the teens. Very similar to RCI's RHAPSODY of the SEAS. Also note that this class is design especially for the Caribbean, meaning younger age cruisers.
I heard from fellow previous HAL cruisers, they said that the ZUIDERDAM is their number unlike ship in the HAL fleet. I guess this explains HAL's idea is working. The number of teens onboard is mainly based on the time you guys are planning. Spring break, holidays, and summer usually has many kids on mainstream/mass market cruise ships.
Now, Celebrity. Celebrity is around the same level as HAL in terms of service. From what I heard, Celebrity usually has a unusual amount of kids on their holiday sailings. Celebrity does not have an amount on kids onboard, meaning you can be stuck with 100 other teens your age.
If I go on Celebrity, I would go only on the Millenium-class. These include SUMMIT, CONSTELLATION, MILLENIUM, and other big one.
Now, it all depends on you. Carnival Cruise Line's passenger are more of the party kind. RCI I thought was ok. NCL has freestyle cruising(dine anytime, anywhere; flexible disembarkation) and a good teen facility on the STAR and DAWN. If you really want to party, go with Carnival.