it is more correct to compare with the height:
(Unit：CM)
90=2T：length=51 hips=72; fits for: 80-90cm height / 2 Years Old

100=3T：length=56 hips=74; fits for: 90-100cm height / 3 Years Old

110=4T：length=61 hips=78; fits for: 100-110cm height / 4 Years Old

120=5T：length=66 hips=82; fits for: 110-120cm height / 5 Years Old

130=6T：length=71 hips=86; fits for: 120-130cm height / 6 Years Old

140=7T-8T：length=76 hips=90; fits for: 130-140cm height / 7-8 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.