I've been on Carnival, Holland America,
Royal Caribbean, Norwegian, Celebrity and the now defunct Big Red Boat.
Holland America is definitely an older crowd, and although the ships are nice, I didn't see them being worth the extra money.
Celebrity has an older crowd too, and it is also more expensive. You will see differences in things like fine china and silver, as opposed to not so fine china and silverware on the other lines. Their entertainment leans more toward travel lectures and less nightlife too.
Royal Caribbean, Norwegian and Carnival are pretty much the same, but it depends on which ship. I've been on 3 different ships from each of those lines, and if you go on the newer ships you will be fine. i would not recommend the older ships on any of those lines. I think Royal Caribbean likes to think they are a notch above, but honestly, Carnival has stepped up it's game with their newer ships and there is no longer much difference.
Carnival has some of the best food, believe it or not. I have only had one bad meal out of 3 Carnival cruises. The problem with NCL is that they are turning towards the "premium" restaurants you have to pay extra for and are not putting much money into the general dining room. They pretty much force you to go to the restaurants that have an additional charge. This can add substantially to the total cost of the cruise. Royal Caribbean is now starting to charge for steak in the regular dining room, whereas the other lines it is included.
A minor point as well, definitely not a deal breaker, is that Carnival has free 24 hour room service, and Royal Caribbean charges for their food between 10 pm and 6 am, or something like that.
Royal Caribbean's newer ships have some great features like skating rinks, rock climbing walls and surfing. The newest ship even has ziplining.
It all depends on what you are looking for. My parents went on HAL to Alaska and loved it, but then again, they are in their late 60's.
If you have any specific questions, let me know. Cruisecritic.com is really a great source of information too.