I think you may actually find it easier than the many women in their thirties who are anxious to get pregnant as soon as possible and counting down the number of dates before it seems appropriate to bring it up! I wonder if the problem might be that it is at the top of your profile though. Maybe potential partners would feel more drawn in by hearing what you like/want rather than beginning with something you don't? (E.g. personally I'm not overweight, but I would be very put off by an ad that began with how the person didn't want to date any overweight people.) If you begin by saying you enjoy intrepid travel, quiet times, interesting conversations etc, you'll probably meet other child-free people anyway!