Celebrity has been experimenting with certain sailings being adults only. Additionally, some of the luxury cruise lines have very few kids. You can also just time your trips to minimize the number of kids (i.e., sail while kids are in school).
Holland American attracts an older less kid crowd. I know that Celebrity is trying a few "adults only" cruise itinaries. Don't travel during school holidays and the summer and stay away from cruise lines like Disney and Carnival - these attract a lot of familes - RCCL isn't bad - but the Voyager class ships attract a lot of familes. Also where you go and for how long makes a difference in the amount of kids - usually a longer cruise 10-14 days won't have as many kids as a 7 day Caribbean cruise. The luxury cruise lines don't get as many kids.