To the OP, please let us know how you made out and whether you were able to do your tour. I am in a similar situation.
I want to take a Catamaran Snorkel tour in Grand Turk. Maximum age is 70. We will be 72 and 73 at the time of the tour. We are both is good shape and are experienced snorkelers. I too went though the booking process on Princess and it looked like I could book it despite our ages. I am very interested in hearing from others who might have done something similar and how it turned out for you.