it is more correct to compare with the height:
(Unit：CM)
80=9M-12M： length=50 shoulder=20 bust=53 waist=51, fits for: 70-80cm height / 9-12 Months

90=18M-24M： length=55 shoulder=21 bust=57 waist=55, fits for: 80-90cm height / 18-24 Months

100=3T： length=60 shoulder=23 bust=61 waist=59, fits for: 90-100cm height / 3 Years Old

110=4T：length=65 shoulder=24 bust=65 waist=63, fits for: 100-110cm height / 4 Years Old

120=5T：length=70 shoulder=26 bust=69 waist=67, fits for: 110-120cm height / 5 Years Old

130=6T： length=75 shoulder=27 bust=73 waist=71, fits for: 120-125cm height / 6 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.