My in-laws like cruising. They are retired and fit in the age group specified - their number one advice when asked is to go with a smaller more luxurios line - dont do Carnival or Royal Caribbean if it can be avoided. They have many reasons for being picky, like having better service, a nice quiet place to have dinner and better rooms, but more than anything they like that there are less kids running arround
A few years ago my wife and I went on our first cruise, going to the Yucotan peninsula on a Royal Caribbean boat and I must say it was pretty close to our vision of hell - but my wife was the maid of honor at a friends wedding so we really couldn't get out of it. From our experience on that cruise I highly recommend one that does not have you eating dinner together with the rest of the 2000 people on the ship. I can think of many horrible things I would rather do than to go through that again.
But alas ... I rant ... long story short: Make sure you get a cruise that pampers to the lifestyle of the people who will be going.