it is more correct to compare with the height:
(Unit：CM)
110=3T-4T：length=61 waist=44 hips=72; fits for: 100-110cm height / 3-4 Years Old

120=5T：length=66 waist=44 hips=76; fits for: 110-120cm height / 5 Years Old

130=6T：length=70 waist=46 hips=78; fits for: 120-130cm height / 6 Years Old

140=7T：length=74 waist=48 hips=80; fits for: 130-140cm height / 7 Years Old

150=8T-9T：length=79 waist=48 hips=82; fits for: 140-150cm height / 8-9 Years Old

160=10T-11T：length=84 waist=50 hips=86; fits for: 150-155cm height / 10-11 Years Old

170=12T-13T：length=89 waist=52 hips=90; fits for: 155-160cm height / 12-13 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.