My 10 year old daughter acts much younger than her age.

Hello. I'm new on here, and would like some advice if possible please. My 10 year old daughter starts secondary school next September and I am concerned that she won't fit in because she acts so much younger than other children her age. She was diagnosed some years ago and she knows that she has ASD, and while it is quite endearing (sometimes) that she is still very childlike, I am starting to worry now that she won't have grown up by the time she starts secondary school, which will make her 'stick out like a sore thumb'. My eldest daughter, who is 15, just keeps saying that she will be bullied, which obviously isn't helpful. Does anyone have any advice please on how we can encourage her to act her age? Thank you.