it is more correct to compare with the height:
(Unit: CM)
5-6Y：Top length=30 bust=76 Skirt length=35 waist=50, fits for: 115-125cm height / 5-6 Years Old

7-8Y：Top length=32 bust=81 Skirt length=37 waist=53, fits for: 125-135cm height / 7-8 Years Old

9-10Y：Top length=35 bust=86 Skirt length=39 waist=56, fits for: 135-145cm height / 9-10 Years Old

11-12Y：Top length=37 bust=91 Skirt length=41 waist=59, fits for: 145-155cm height / 11-12 Years Old

13-14Y：Top length=40 bust=96 Skirt length=43 waist=62, fits for: 155-165cm height / 13-14 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.