The recommended age does not necessarily apply to everyone.
The fit varies depending on the height, weight, and body type of the child

S (6-12m) : Chest: 22inch all around 

Length: 16inches

M (12-24m) : Chest: 24inch all around 

Length: 17inches

Baby/kid age is an estimation of fit, not all babies/kids are of the same size, so please keep in mind dimensions of clothings, rather than age groups for a more accurate fitting