We like NCL , and our teens loved it. It's very relaxed, no need to dress up unless you want to - one or two of the restaurants require long trousers and a collared shirt (polo shirt is fine), but that's about it.
You can also eat when you want and with who you want, no need to table share with strangers. You don't need to make restaurant reservations (except the specialities), you can just turn up when you want to eat, you'll be given a buzzer if there's no table immediately available, but we've never waited more than a few minutes, even at busy times.
Depending on the ship, there will be a variety of things for teens to do - sports courts, water slides, bowling alley, arcade, mini-golf, table tennis etc. Can get quite busy, especially in school holidays. All will have a teen club if your teens are under 18, and some have a 'meet up' for 18-25 year olds if they are over 18.