NCL has a GREAT kids program whether young or teens. As the other posters stated, you may not likely see your teens to much. Many teens make new friends, hang out together and have lots of activities that they do together. There are lots of video games on board in special rooms designated for the kids program. NCL designs special programs for all ages, and there are lots of places on the ship for teens to explore, and eat together.
I wouldn't worry about my teen children being bored. I am sure they just might find a cruise on a ship one of the "coolest" vacations ever.