Don't stress out about choosing the right ship. There are dozens out there that would be great matches for you. A 10 day cruise will probably have a bit older crowd than a 7 day. Not to say that the young-at-heart can't be as lively, or high energy, as you described.
If you are interested in snorkeling, you might consider the Caribbean over the Mexican Riviera. And definitely come back here to help you plan your time in port for lots of do-it-yourself tips and suggestions.
Personally, I would begin choosing a cruise by itinerary. Then cruiseline and ship.
Fire away with questions!