In my opinion, girls develop and mature greatly between the ages of 10-15. It seems like every "six month" in age makes a difference as they change so fast during this time frame. I would guess that the jump from 10u to 12u is probably more difficult for pitchers as they are changing pitching distance (35 ft to 40ft) and also going from an 11" ball to a 12" ball. The size difference, including physical maturity, in this age group is amazing. I watched the Silver Creek Starz play the Lasers Red in PA and at the end of that game, the tallest girl from the Red (5'11" i'm guessing) stood next to the shortest girl on the Starz (4'11") and these two posed for a picture together.