My son has sailed the Magic at 11 1/2, 13 1/2 and coming this August - 16 1/2. It's his favorite vacation destination.
He enjoys the planned activities and also the freedom.
He liked:
Hanging out in the stack and sports deck
organized games (boys vs girls trivia type games)
the kids on their own had a 'pizza party' one night poolside, with pizzas from Pinnocchio's
Teen "Wild Side" excursion on Castaway Cay - snorkling, biking, kayaking
pool table, ping pong, and other stuff with the teens at Castaway Cay
He'd make some friends, they'd go to lunch every day, etc. He's not rowdy; is more on the serious side too.
Take a look at the Teen itinerary on some recent Navigators to see if he'd enjoy the planned activities - Good Luck!