My 10 year old daughter is a size XS women's or a 00 at Aero. She is 5'7" tall and the pants are the hardest things to buy. Sizes which are long enough are built for a girl/women who has curves. At 10 years old, she is still a little girl trying to fit into adult sizes. We have been lucky that Aero works and she even fits into Holister XS clothes now too as well as Victoria Secret's "Pink" leggings. Like others have said, each brand will be different, so it may take some shopping around.