Can a 9 year old girl act a 13 year old girl?
It depends. If shes a good actress and lots of experience, and if the script would work then yes, probibly. It also depends on her body, and looks, she needs to look like a 13 year old.
If you are NOT a 13-year-old girl, then you probably don't need to act like one. If you ARE about 13, you could try watching, then acting like some other 13-year-olds. If you feel comfortable with their behavior. If your a 13-year-old, or any other age for that matter, be yourself, but in a mature way!