If there is an age limit it generally is strictly enforced and it won't matter if your daughter looks older then she is because they have everyone's birthdate from their birth certificate or passport. That is also how they make sure the kids are in the right group at the kids program, there is a master list they get with all the birth dates.
The stingrays are great in grand cayman, make sure to bring an underwater camera. As long as your daughter is good at walking, some kids are very whiny about walking 30 minutes carrying a tube I think she would love the cave tubing check out www.regtour.com
we went cave tubing with him tour instead of the ship's and had a great time for $20 less/person then the people on the ship did.
Our friends did the sub tour in Cozumel and said their kids would of loved it but it is expensive. The Fury Catamaran is fun for everyone in Cozumel. There is alcohol served on the boat but there were plenty of kids on our sailing and no one got out of control.