Is it normal to have size 36DD bra at 14?

Yes, I had that at 14 and by the time I was 18 was 38DD so don't be surprised if you don't add a few inches by 18. My answer to the size was to hide in shirts that were too big. Don't let men/boys be rude to you ( they will) so think of a good comeback to tell them. I have one, but I can't say it here. Knocks them right down.