We loved out cruise on the Rotterdam in June very relaxed, however our most memorable cruise was the 15 night Transatlantic Crossing on the Carnival Legend in Sept 2002. Hubby and I are in our 40's but the crowd on this sailing was on average, somewhat older. The service and food were wonderful and the itinerary was perfect.
It it certainly not fair to generalize about one cruise line. I know that the older Carnival ships attract the party crowd, especially in the caribbean, however the newer Carnival ships on the longer itinerary can be a great experience. Having said that we are currently booked on 2 more HAL cruises- one we one on board the Rotterdam and the other one was booked for the amazing itinerary(Westerdam 17 nights Nov 12 2004-Farewell to Europe) We do plan to continue sailing on both lines.