My DD started pitching at 9.5yrs. old last March. Pitches that actually made it to a catchers glove then was averaging like 30-33. We started with an instructor in late May, accuracy increased significantly as did her speed. With improved technique and accuracy, came speed. By summer ball she was averaging 35-36, end of summer her strength had grown and she was averaging 37-38. She set the goal of 40 by end of fall. By late September she hit and passed an average of speed of 40 seeing more of an average of 41-42 when she turned 10yrs. old that month. Now we are targeting 45 by the beginning of Spring Ball.
All this is strictly her fast ball. Along with this is we continue to develop her change up and one more pitch. Plus, we continue to work accuracy with speed. What good is 45MPH if it is in the dirt or fence.
I also take averages; has she hit 45, yes like 3 time out of hundreds of pitches.... I am not her claiming my kid can throw 45. I think averages is the more important measure.
Now, that said you are going to hear a bunch of people tell you outrageous numbers the classic "My girl is 8.5 and throws 54 mph!" Don't let that detour you or her in her goals. Set a goal and work at it. Whether that is accuracy, speed, a new pitch, a better change up.... strength.....
That said, our #1 pitcher is basically the same age as my daughter. She started pitching a year before us. By end of fall ball, we were measuring her at 44-45 average and I radared her personally to see a 47 one night at practice. The draw back is she can be a bit wild, but when she is on, it is cool to watch. My Daughter uses her as someone to chase, which just makes the whole team better.