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: 14inch all around before elastic stretch, stretches good 5inch both ways. 

Length: 18inches

M (12-24m) : Chest: 20nch all around before elastic stretch, stretches good 5inch both ways. Length: 19inches

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