it is more correct to compare with the height:
Unit：CM
80=9M-12M：length=66 bust=69 sleeve=30 shoulder=24; fits for: 70-80cm height / 9-12 Months

90=2T：length=70 bust=75 sleeve=33 shoulder=26; fits for: 80-90cm height / 2 Years Old

100=3T：length=74 bust=85 sleeve=36 shoulder=27; fits for: 90-100cm height / 3 Years Old

110=4T：length=78 bust=87 sleeve=39 shoulder=29; fits for: 100-110cm height / 4 Years Old

120=5T：length=82 bust=93 sleeve=42 shoulder=30; fits for: 110-120cm height / 5 Years Old

130=6T：length=86 bust=99 sleeve=45 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.