Of course, every teen is different, my boys (14 and 15) still liked some of the Disney-ness when we were on the Fantasy this summer, but there were lots of other things that appealed to them about the cruise. Free food, as often as you want in various venues. Free room service. (Normally, I NEVER let them order room service). Free ice cream. Free soda. OK, i just said free food four different ways, but to my growing boys that was four separate selling points. Not only that, but I felt comfortable giving them much more free reign on the ship than I would have on other vacations. They spent a ton of time in Vibe, or on their own on the Aqua duck, or the sports deck, getting their own food, or hanging around the ship.
They LOVED the freedom, and yet, with the Nav app, and the fact that the ship isn't THAT big, I could always find them when I wanted to. But that doesn't mean I never saw them. Because of this freedom, they were happy to eat with us, and go on excursions with us, and even see some shows with us.
I know the OP said her sons had been on a cruise before, so maybe the novelty has worn off, but maybe the trip just needs to be packaged differently. Plus, I definitely sold this trip as an opportunity for them to show me that they were ready for more freedom/responsibility. "Show me that you will stick to your word, go where you say you are going to go, and return when you promise, etc. so that I know that I can count on your following your word in the future, and thus give you more freedom/responsibility. My older son will be getting his driver's licence next summer, so the Freedom/responsibility balance is definitely a big thing for us, and this cruise showed us that he is getting ready for that.