it is more correct to compare with the height:
(Unit：CM)
90=12M-18M：Shorts length=29 waist=23 hips=30, fits for: 80-85cm height / 12-18 Months

100=18M-24M：Shorts length=32 waist=24 hips=32, fits for: 85-90cm height / 18-24 Months

110=3T：Shorts length=35 waist=25 hips=34, fits for: 90-100cm height / 3 Years Old

120=4T：Shorts length=38 waist=26 hips=35, fits for: 100-110cm height / 4 Years Old

130=5T：Shorts length=41 waist=27 hips=37, fits for: 110-120cm height / 5 Years Old

140=6T：Shorts length=44 waist=28 hips=39, fits for: 120-130cm height / 6 Years Old

150=7T-8T：Shorts length=47 waist=29 hips=41, 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.