Welcome to cruise-chat.
As we often tell members, it is hard to say you will like a particular cruise line without knowing you. Shorter cruises (3 and 4 night) tend to attract a younger crowd more concerned with packing a lot into a few days, as opposed to the more relaxed nature of a longer cruise. However, Carnival caters to a broad demographic and especially in late January you will probably find fewer kids and families, and more couples. 57 is definitely not old for Carnival.
Sensation is one of several mostly identical Fantasy-class ships. For a first cruise you will most likely be wowed by any ship you board since you have nothing to compare it to. Carnival's Fantasy-class ships, such as the Sensation, are often described as having "loud" decors in a Las Vegas style so you might find it familiar.
The important thing is to have your expectations properly set and not expect more than what Carnival can deliver. Do not expect 5-star cuisine and waiters parading into the dining room carrying platters of huge lobsters, ala Love Boat. It simply isn't so. Do expect very good food, and often excellent, served by a gracious and well-trained staff who will bend over backwards to make your cruise memorable. Carnival has excellent buffet restaurants with 24 hour pizza (very good), a NY style deli with excellent sandwiches, usually an oriental station, and of course the usual buffet fare. All in all, I'd say it is at least comparable to Vegas food.
You can do as much or as little as you want on a cruise. The daily schedule will be packed with things to do...and to be honest a lot of them are focused on extracting money from you. You need to realize that sodas, liquor, bingo, the shops, the casino, etc., all cost extra. It is often true that your onboard account total will equal or exceed the price you paid for the basic cruise.