I never got the whole "Don't say that/Don't talk about this anymore," attitude personally. It's been said, and it doesn't stop being true just because it's not being said. I've had that attitude used on me in the past with something else but just as major, and it sucks, and there's nothing worse than that feeling of being so alone because of it.
I'm sorry you are getting this from your parents, especially when it comes to something as major and as severe as Gender Dysphoria. It's unfair and horrible and sucks. But you're not alone. You have us, and we are here for you whenever you need. Both in post and PM.
I really hope your parents have a change of heart, but even if they don't, like everyone here has said, once you're 18, you are able to decide yourself. Don't let them keep you from being yourself and being happy. As soon as you are able, take the steps you need to to become your true self. And never hesitate to use us when you need a shoulder or ear or just a reminder you're not alone.