It depends on the cruise line and when you go. My kids ( ages 7 & 8) will be going on their 4th cruise this March 2000. We're doing the Voyager and I expect it to be full of kids running everywhere since it's Spring Break and such an adventurous boat. We did the American Hawaiian Independce last spring and the Celebrity Galaxy the year before. There were a few kids on each boat, but they have a wonderful kids program to keep them entertained throughout the day and the price of both cruises might be a bit prohibitive for a lot of families, so the kids were all very well behaved.
If you don't want to be bothered by kids, I would suggest going on a more upscale cruise line! With more and more families traveling to places that used to be considered "exotic" destinations, we find traveling at times when most kids are in school gets us away from them.