it is more correct to compare with the height:
(Unit：CM)
Size 3=5T-6T：Vest length=12 bust=64 Coat length=41 shoulder=46 sleeve=37 Pants length=75 waist=49, fits for: 120-130cm height / 5-6 Years Old

Size 4=7T-8T：Vest length=12 bust=68 Coat length=43 shoulder=48 sleeve=40 Pants length=80 waist=51, fits for: 130-135cm height / 7-8 Years Old

Size 5=9T-10T：Vest length=13 bust=71 Coat length=46 shoulder=50 sleeve=43 Pants length=85 waist=53, fits for: 135-145cm height / 9-10 Years Old

Size 6=11T-12T：Vest length=13 bust=75 Coat length=48 shoulder=52 sleeve=46 Pants length=90 waist=55, fits for: 145-155cm height / 11-12 Years Old

Size 7=13T-14T：Vest length=13 bust=78 Coat length=50 shoulder=53 sleeve=49 Pants length=95 waist=57, fits for: 155-160cm 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.