I was told that i have a septate hymen by an emergency room doctor after getting a tampon stuck. however, she said that gynecologists dont usually do hymenectomy for women under 18, is this true?

Maybe: I suspect the age factor is more likely related to the age of consent in your state. Most allow a limited number of health interventions without parental notification and consent. This would be an elective surgical procedure and likely requires parental involvement unless you are an emancipated minor or married. If consent is granted. I feel many OBGYN's would do it.