I'm just the teen host on here. I don't actually work for a cruise line, although my experience with the various lines is quite extensive. In any case, age is usually taken on a case by case basis. Try talking to the O2 director onboard on your first day on. If there are any issues, simply tell the director exactly what you just told me; you're a high school student, sailing with your cousin, and you'd appreciate some leniency. Of course, this all depends on how tightly the director follows the rules. I've had some that are super strict, and others that were super lenient. It all depends on the sailing and the crowd.
Let me know if you have any other questions,