it is more correct to compare with the height:
(Unit：CM)
73=6M-9M：fits for: 55-65cm height / 6-9 Months

80=9M-12M：fits for: 65-75cm height / 9-12 Months

90=2T：fits for: 75-85cm height / 2 Years Old

100=3T：fits for: 85-95cm height / 3 Years Old

110=4T：fits for: 95-105cm height / 4 Years Old

120=5T：fits for: 105-115cm height / 5 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.