My daughter is 12 and she can run a 6:39 mile but she is pretty fast for her age. She also plays travel soccer and that requires lots of conditioning and running. Even though she is pretty fast, I know for a fact that she is not the fastest girl at her school that is her age, but that is the fastest that I've heard of for a mile time.
I am 11 and I personally don't think that I run fast. In P.E., we do mile runs almost every time (every 2 days) and I feel like people are definitely getting better. It really matters on how fast you are and how much strength you have. Here is the average for boys and girls in my P.E. class. Boy:8 minutes Girl:6 minutes
I think that an average time for a ten-year-old girl to run a mile is about 7 minutes to 10 minutes. 7 for fast runners, 10 for, not slow, but people walking or jogging. Don't be ashamed if you got more than a 10 minute, or if you got less than a 7 minute, wow, your good! But it also matters about your training level, weight and height.