You may not be underage, but that doesn't mean covert ageism will not be an obstacle. I would personally try to get help without specifically mentioning that you are
TTC. Do you have irregular cycles, or any other symptoms that something could be wrong? Bad pain on your period? Acne? Short luteal phase (time after ovulation)? You're going to have to be your own advocate, so I would start reading as much as possible and chart your cycles with your basal body temperatures in order to get to the bottom of this. Start by checking out
Taking Charge of Your Fertility at your local library, that is basically all the information you need to know in one place. Good luck!