This is something that I have not seen addressed yet and thought maybe it should be.
The longer the cruise, the more expensive it is. And people who work and/or have kids frequently can't do more than 7 days at most. In my experience, if a cruise is longer than 10 days, the passenger mix tends more toward older and more upscale. The two together mean dressier attire, especially at dinner. Just FYI to anyone. It's not just the cruise line. A 16 day HAL cruise is different than a 7 day HAL cruise (loved 'em both). And, a 3 day Carnival, for example, is entirely different than either.
We also seem to only talk about certain cruise lines...Celebrity vs. Oceania, for example. There are a lot more lines where things are fuzzier re. standards of attire. And, every cruise is different. Having lived most of my adult life in either Florida or L.A., I have frequently done the same cruie line, same ship, same itinerary multiple times. And, one time the passenger mix would tend toward quiet and well dressed, the next would be the bucket of beer crowd who don't think anyone should wear anything besides jeans and baseball hats. At least they were required to take those caps off for dinner! I stopped doing weekend getaways via 3 day cruises because of the rowdy groups that often tend to book those. They are more than welcome in my book to enjoy themselves. I just don't want to go along.