it is more correct to compare with the height:
Unit：CM
70=6M-9M：length=42 bust=48 waist=44 shoulder=17, fits for:65-75cm height / 6-9 Months

80=9M-12M：length=47 bust=52 waist=48 shoulder=18, fits for:75-80cm height / 9-12 Months

90=18M-24M：length=52 bust=56 waist=52 shoulder=19, fits for:80-90cm height / 18-24 Months

100=3T：length=57 bust=60 waist=56 shoulder=21, fits for:90-100cm height / 3 Years Old

110=4T：length=63 bust=64 waist=60 shoulder=23, fits for:100-110cm height / 4 Years Old

120=5T：length=68 bust=68 waist=64 shoulder=24, fits for:110-120cm height / 5 Years Old

130=6T：length=73 bust=72 waist=68 shoulder=26, fits for:120-130cm height / 6 Years Old

140=7T-8T：length=78 bust=76 waist=72 shoulder=27, fits for:130-140cm height / 7-8 Years Old

150=9T-10T：length=83 bust=80 waist=76 shoulder=29, fits for:140-150cm height / 9-10 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.