We have cruised 20+ times, mostly with Carnival, Royal Caribbean, and Norwegian, but have also been on
DCL, Princess, Celebrity, and a couple others. Celebrity and Princess were definitely older crowds, but we loved the educational seminars. DCL had characters, but at 2-3 times the cost for a comparable cruise and the experience was not 2-3 times better
Carnival, RCL and NCL were all comparable with newer ships on all three generally better than the older ships on all three. The 3-4 day voyages tend to have the newest staff and most turnover, so they tend to be the most hit or miss on all three lines. The week long sailings on relatively new ships tended to be the better staff, food, and entertainment on all three.
RCL is the best for kids hands down. In our experience, their shows are more kid-friendly than any other line (except DCL). They tend to show family-friendly movies and have lots of activities going on throughout the day. Our kids love the rock walls, ice rink, hot tubs, movies by the pool, water slides, game shows, and food options.
Carnival is a great option as well and usually has the most choices for different ports and sailing dates. The shows and movies aren't always appropriate for kids, and most of their ships don't have as many activities on board for families, but we all have a great time anyway. We usually choose cruises based on ports we want to try and often have very limited travel dates, so Carnival gets a lot of our business.
DH and I alone enjoy NCL, particularly for the live music, dining options, and free drink packages! Unfortunately, the shows are not family friendly with costumes and choreography I don't want to explain to my 9yo with special needs, and they tend to be more disco and 70's themed, so not particularly appealing to any of us. We often find ourselves bored with nothing of interest going on anywhere on the ship--this happens on other lines too, but I think more with NCL. We are going to Hawaii next winter on NCL so hopefully that ships will cater more to families. We haven't been on their newest ships, so this might not be a fair assessment as those seem to be targeting more of the family crowd.