Hi,
Generally the spa is 18 and up. According to the eastern, they do have a teen time. For the Eastern it says 8:00am-8:00pm on the the St. Martin Day and the St. Thomas Day. It specifically says: Enjoy a mini-facial, mini-manicure and a mini-pedicure for $69. Booking required.
It appears that this is the only time and service. perhaps someone else can add to this. I checked my Wonder navigator and it did not list any time.
You might also check the 4-night wonder navigator and the western navigator for those times as well.