it is more correct to compare with the height:
(Unit：CM)
80=12M：fits for: 66-73cm height / 9-12 Months

90=18M：fits for: 70-80cm height / 12-18 Months

100=24M：fits for: 80-90cm height / 18-24 Months

110=3T：fits for:90-100cm height / 3 Years Old

120=4T：fits for: 100-110cm height / 4 Years Old

130=5T：fits for: 110-120cm height / 5 Years Old

140=6T：fits for: 120-130cm height / 6 Years Old

150=7T-8T：fits for: 130-140cm height / 7-8 Years Old

160=9T-10T：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.