Back in the day, when cruises first made their appearance, they did cater to the wealthy and retired, but that is definitely not the case any more. People of all ages are cruising, including families with children (Disney cruise lines are popular with this group) and singles.
There are cruises for virtually anyone no matter what your interests.
Short cruises in the Caribbean or down to Mexico (often wrapped around a long weekend) are particularly popular with young adults who want to get away and party, party, party for a few days. If you want adventure, you can find ships that will make stops in the perfect spots for SCUBA diving, rock climbing, kayaking, hiking, and other active pursuits. If the idea of learning something new on vacation excites you, you can find educational cruises featuring guest lecturers who will share the history of the region or even help you become a pro at digital photography.
By now, you should see that this cruise myth is definitely false. There are plenty of things to do on cruises for people of all ages, and cruising is one of the best vacations you can get for your money (and you don't have to spend a ton to have a really good time).