I think the cruises that offer bargain fares seems to attract a more senior crowd. My sister went on the Legend last January 2003, and said the ship was almost all seniors ( age range retired). They got a great fare thought, but her two 18 year old sons were misersble (very few teens).
My family went on the Legend, Presidents Day week Feb 2003, and there were lots of families, and very few seniors. I would say the average age range of adults was about 23-50. This week is also a peak week for fares, and fares were double what they were in early January.