My parents take 2 -3 cruises a year and are very partial to Celebrity. My parents are in their 70s, but like Celebrity because their shipmates are generally not a bunch of doddering geriatrics... it's all ages and very few children. The boats they've been on - which I beleive leave out of puerto rico, go all about the carribean- they rarely will get off the boat but it's avaialble if they want to... they like the quietness of everyone gone.
There's a thread or two on cruises in here - especially on whcih cruise line is right for you - I'm too lazy to search for it...
Generally when the price is quoted as per person/double occupancy - it's double the price quoted.. (and doesn't include tips - whcih are pretty much required) some cruiselines also charge for booze - which can really add up...
Check out some websites like
www.icruise.com for some ideas...