If you're interested in going on a cruise with more adults and less kids, then use this general guideline to help in planning;
1) Choose a more upscale cruise line. For the most part, families want a cheaper cruise with lots of amenities for the kids. Of the mass-market cruise lines, Celebrity and Holland America normally draw an older crowd and their costs aren't much higher. Any of the luxury cruise lines (Crystal, Regent, Seabourn, Silversea, etc.) will always have very few children at any time of the year because their costs are much higher and few families can afford these. This is why you'll see more families on Carnival, NCL, RCCL, and Princess than any of the other cruise lines and explains why their ships are designed accordingly. Any of the more upscale or luxury lines have very few, if any, amenities for children or teens.
2) Choose a longer cruise. Families like to stick with 7-nights or less because of their schedules and budgets.
3) Go to someplace different. For the most part, families like warm weather and nice beaches. Most kids don't necessarily like places like Alaska, Canada/New England, or Baltic cruises.
4) And the obvious one, don't choose a cruise when school is out.
There are always a few exceptions to every rule and you'll almost always find children on just about any cruise at any time of the year. (Home schooled families love to travel during the off-seasons.) But having been involved in travel for over 35 years, these guidelines apply to the majority of situations where people want to travel when families are not traveling.
Conversely, if you want to travel when there are plenty of kids and teens, then travel anytime school is out. You're almost guaranteed plenty on just about any ship, especially during the highest traveled times of the year; Christmas/New Year's, Spring Break, and the first couple of weeks during Summer are the most popular, but will also demand the highest prices.