I'm the family cruise editor here. While Carnival does have activities just for 16 and 17 year olds, it is not as extensive as for those 15 years and under. Basically the social and activities director of each ships sets up things for the 16 & 17 year olds to do, such as attending a show together or going to the disco at a special time together. Their activities are not overseen by a youth counselor, as are the activities of those 15 years and under.
If you're looking for lots of options for teens through 17 years, check out Royal Caribbean's Navigator of the Seas which is setting sail for the first time any day now. This ship will have the most extensive teen facilities ever at sea, ranging from teens only outside deck space, teens only fast food areas, and teens only discos, plus more.
Let me know what you decide.
Smooth sailing,
Luisa Frey Gaynor