They never believe I am 14!!!!!?

I am a very tall girl and I am 14 but I am in grade 7 and I act very babyish but now I am trying to act a bit mature but in my school all the girls in grade 8 and 9 are between the age 13-15 and when we talk to them (me and my friends) they say you wouldn't understand you are too small so i just go like btw I am 14 and they get shocked and go like OMG wow they think I am a baby even my cousins