it is more correct to compare with the height:
Unit：CM
90=18M-24M：length=38 bust=70 shoulder=26, fits for: 80-90cm height / 18-24 Months

100=3T：length=41 bust=74 shoulder=28, fits for: 90-100cm height / 3 Years Old

110=4T：length=44 bust=78 shoulder=29, fits for: 100-110cm height / 4 Years Old

120=5T：length=47 bust=82 shoulder=31, fits for: 110-120cm height / 5 Years Old

130=6T：length=49 bust=86 shoulder=32, fits for: 120-130cm 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.