There is no easy answer to your question. There are a few lines that are fairly predictable; the oldest passengers are usually found on Holland America. The Spring Break / party til you puke crowd is sometimes on Carnival. But aside from that, most every cruise on most every cruise line is different from all the others. I am taking my 1,018th cruise this week (you may guess that I have worked on ships for a long time). Almost every cruise was different and unpredictable. Of course Christmas cruises are usually the worst ones of the year; New Years cruises are usually the best. Family holiday cruises (Thanksgiving, Hannukah, Christmas, amd Easter)are usually terrible for crew and passengers - but not always.
If you want to be over-run with small children and families with no money, take a cruise near the USA during school holidays. If you want an older crowd, get further away from the USA and/or take longer cruises. Young families with children usually cannot afford the time and money to go further and longer. Older retirees usually have the money and the time for longer exotic cruises.
The average age of passengers on mass market ships can vary - in one week - from 40 to 60 years of age. Remember, that`s AVERAGE. Most cruise lines and most ships do not know what the average age for a cruise will be until a few days before the cruise starts.
When you take a cruise, you get just a snapshot of a ship and believe that the ship is always like that. This is rarely the case. The week you are aboard, there may be a really boring group sailing. The next week on the same ship could see a really wild party crowd.
To increase your chances of a good nightlife crowd:
Avoid family and school holidays.
Look at New Years and Spring Break cruises.
Depending on what you are into, gay cruises are usually really outrageous - as in fun.
Avoid long or distant cruises.
Do your homework in the chat rooms. Some ships and some lines attract a younger party crowd - but not always.
Remember that cheap cruises attract cheap cruisers.
Good luck - and have fun.