Barbara, welcome aboard, and congratulations on your 50th wedding anniversary! That is a milestone to be celebrated in a big way, and taking your family on a magical Disney Cruise is just the way to do that!
Unfortunately, the age restrictions on the Port Adventures are very strict and not flexible. If your granddaughter will not be five years old before the cruise, she will be unable to book and go on that
Fury Catamaran Sail, Snorkel & Beach Adventure. I don't know what the restrictions are through the Fury website, but I always recommend booking adventures right through Disney, as they take care of all the details. You can be sure that even due to a ship delay, you won't miss your adventure, nor miss the ship due to an adventure coming back late.
You might consider taking a look at the Port Adventures, sorting them by "Family Adventure" and then note the age restrictions on each. I'd highly recommend the
Cozumel Beach Break for your family with a five year old. I was just on the Magic in the Western Caribbean a few months back and my five year old son absolutely loved that adventure. There are plenty of loungers for adults to relax on the beach, great food and drinks, and tons of activities for kids from babies to teens. Your granddaughter would really like the Buccaneer's Bay Swimming Pool and the water slides there. It's a relaxing afternoon or an event filled day, it's really up to each guest to do whatever they feel up to and there are so many things to choose from at Playa Mia.
I hope that helps as you continue to plan your cruise, Barbara. Please stop back with any other questions, as I'm happy to help!