You have to be 18 to enjoy the spa BUT on port days children can enjoy a variety of treatments (manicures/pedicures/facials). However, you must be 18 to experience the wonderful massages. Lots of kids were disappointed about that.
If you go to "The Stack" (the teen hang out) you can work out in the gym anytime after 10 p.m. except on the first and last night of the cruise.