I have 11 year old twins that wear these for cheer/tumbling class. My girls are on the lean side, so fit & coverage is fine. One of my girls wears a 10, so I ordered the medium and the other (taller) wears a 12 , so I ordered her the large.
What I did NOT like was how quickly it showed wear. After 4 washes, the bottom is completely pilled. Literally looked like a swimsuit that you sat on the concrete all summer. Except that these are worn inside a gym, with no concrete surfaces.