Sara,
You may want to verify the age with Carnival for the Pirate Excursion in PV.
I went on it with my family in September 06, and my son was 9 at the time. It had some snorkelling, fun on a secluded beach, and cruising on a pirate ship with breakfast if I remember right (I know there was a mean involved).
He'd have fun...and so would you!