Ages are approx.
Small: 6-12 months
(but can sometimes fit newborns! Super stretchy!)
Medium: 12-24 months
Large: 24-36 months
These are very stretchy and fit older than the approx ages shown
Even children up to teen, they will just be shorter style on taller children.
Children come in all shapes and sizes, so ages are approx. If you are doubting fit, go with the LARGER size.