I have found that January through December are excellent times to cruise.
Seriously: It depends on the itinerary. If you are cruising South America then it's November through March.
Caribbean: I find that late fall, other than holidays, is a great time. Lower prices and less chance of bad weather.
Europe is basically a spring and summer schedule.
I find that the longer the cruise the less number of children. Also, cruises outside the Caribbean have fewer children.
Mike (Who believes that there's no bad time to cruise)