Unfortunately, Carnival, like Royal Caribbean has cracked down on their policies as a result of a LOT of problems with younger and older teens causing problems in the clubs. Regardless of whether you choose Carnival, Royal Caribbean or any of their respective affiliate "sister" lines, they have a policy they follow. If you are within one month of your 15th birthday, then on RCI you must complete 2 Navigator Activities (12-14), and have your parents sign a waiver and then make the decision to move up or stay where you are. Carnival has the same date range (1 mo) but you are generally allowed to enter without being required to go to a Circle C activity. Please remember, however, that the decision to admit anyone who doesn't immediately meet the requirements, is left solely up to the teen club director/manager. If you are more than 3-4 months from your 15th birthday, then the chances of you "legally" being admitted are far slimmer. Royal Caribbean is implimenting the SeaScan on all of their ships for both adult and teen clubs. This device scans your Seapass card and immediately explains your elgibility (or lack thereof) for that club. Carnival is mandating that all guests participating in the Club O2 program have their Sail and Sign Card stamped with the Club O2 sticker and they promptly check them in the club. Again, the best solution would be to talk to the director when you board as each has their own criteria on their ship.
Best of Luck