The sizes of the patterns Puperita are based on measurements for children in the 75th centile meaning they will fit 75% of children in that height range (height is always a better indicator than age) which is why you should always go by measurements never age.

This list of SIZE CHARTs includes a chart of comparison between the Puperita sizes and the Ready To Wear sizes.

RTW (Ready To Wear) sizes vary significantly both between manufacturers and internationally.