Cruise at certain times of year
The first obvious point would be to select a cruise during the school year, when children are not on break. This is still not a 100% guarantee of no children, as many parents will take their kids out of school if the price is right. Typically if you stay away from holiday sailings (Christmas, New Years, Easter, Thanksgiving), Summer and Spring Break (Mar-Apr), the number of kids will be greatly reduced.
Stay away from shorter cruises
Another point will be the length of the cruise. Shorter cruises, especially over weekends, will draw more families with children. Try to take an “off-season” cruise of 7 days or more. Cruising for 10 days or more will almost ensure a child-free cruise.
Itinerary
Itineraries such as Panama Canal, Alaska, Europe and Exotic Caribbean will not draw as many family focused cruisers.
Avoid certain cruise lines
Stay away from cruise lines that cater to families with children, specifically Disney Cruise Line, Carnival Cruise Line, Royal Caribbean and Norwegian Cruise Line. Other major cruise lines like Celebrity Cruises, Holland America and Princess Cruises carry far fewer children throughout most of the year.