The sizes are; S (Size 7-8)|M (Size 10-12) |L (Size 14-16) |XL (Size 18-20|http://www.whitesierra.com/media/sizecharts_med.jpg
The size is not measured by ages. The size is measured by waist/chest/bust/etc. For example, a small would be a SIZE 7-8 and a large would be a SIZE 14-16. You have to check the sizing guide and your child's measurements and see which match up.