OK this is how the billing works:
When you go to a dr's office and you are underage, you have to list one or both of your parents as a guarantor. It does not matter whose health insurance you are using. You will be billed for any remaining balance unless your parent(s) are listed as the guarantor. For example, let's say i was on my spouse's insurance. If I see a doctor and there is a leftover charge, the bill comes to me, unless I list my husband as the guarantor of my account.
Your doctor's office should have done this for you in the first place as an underage patient. You might call them up, ask them to tell you who is listed as the guarantor of your account, and see who was listed. If your parents were listed, getting this credit problem alleviated should be fairly simple, in that all you would have to do is file an appeal with the credit reporting agency. As for the charge when you were 19, Can you call the gyno's office, explain what happened, and offer to set up a payment plan?
I hope this info helps.