We are taking our 2 daughters age 16 and 17 on the Istanbul/Barcelona cruise in July. I'm sure there will be other teens on board, our daughter has already emailed a 19 year old guy. Several people seem to be taking college age kids on our cruise. Even if there are not many, you only need a few, and they find each other without supervision on a ship. Good luck, hope you are on our cruise.
My experience i s that -- they will be too old for the teen a ctivities--
They WIll meet others their own age
They will try to get in the casino(not allowed for the 17 year old, but ok for 19)and the bars.
ANd they will want to drink (probably alot)
They will have a Great time