I was on vacation in Alaska once, when I was 31, and I took a day cruise to see the glaciers. It turned out to also be an excursion that was offered to Princess cruise passengers. I remember watching the bus unload and being shocked at how everyone was in their 60s and 70s. I decided I would never cruise with Princess until I was at least that age, since it was clearly an older crowd who opted for Princess, at least for Alaska.
A friend of mine cruises quite a bit, having worked in the industry, and her favorite cruise lines are Celebrity, NCL and MSC.