We carry our 20-year old full-time college daughter on our insurance, but in her two years of college thusfar, her only medical expense has been one trip to the school health center for a minor illness. Was it bronchitus? I can't even remember. Admittedly, she's been very healthy, and this hasn't been a problem.
Once she graduates, I anticipate she'll have her own health insurance (and car insurance). I can imagine us continuing to carry her for a few months while she makes the transition from student to worker, but I don't expect to do this indefinitely, nor do I see us continuing to carry her until she's married, pregnant, has children. She's a healthy, smart, capable young woman, and she'll be able to take care of herself.