Other people have answered correctly: the longer the cruise, the older the crowd. That's mainly because of two things: time and money. Older people generally have more of both than younger people do. The younger crowd prefers the "fun" cruises, mostly Caribbean routes, ones they can get drunk and tan on, and they'll do some shopping at good ports, too.
Also, the farther away the destination is, the older the crowd. Because of price, too. Most 20-somethings can't afford to fly around the world to Australia for a vacation. Of course, there may be 20-something Australian people on that ship, but generally, it will be an older crowd.