it is more correct to compare with the height:
(Unit：CM)
70=12M：length=61 bust=67 sleeve=26, fits for: 70-75m height / 12 Months

80=18M：length=64 bust=69 sleeve=27.5, fits for: 75-85cm height / 18 Months

90=24M：length=67 bust=71 sleeve=29, fits for: 85-95cm height / 24 Months

100=3T：length=70 bust=73 sleeve=30.5, fits for: 95-105cm height / 3-4 Years Old
Note:
1.There is 2-3% difference according to manual measurement.(1 inch = 2.54 cm)
2.All measurement in cm, please note 1cm=0.3937ch, 1 inch=2.540cm
3.Please check the figures carefully and order the right size, don't completely depend on the " age advice". It's just for reference.