My little one wore a 24 from 3yo (little big and sleeves turned up) to now as a 6yo. She's growing out of it now, but is a little big for her age. I'd say 5-7 ish depending on the kid?
(new jacket is a 28 because it's a more fitted style, and I'd say she's solid but not fat)
Heavans - my 9 year old daughter is still in 24"s. I bought her a 26 and she's swamped! But she is small, slender and light.
Seriously - it can depend on the make and definitely depends on the breath of shoulder. Tagg come up quite small, Dublin and Shires come up bigger. The best way I found it to try on everyone elses and find a make which suits then stick with it! You can get great jackets off E Bay as kids keep growing out of them - but you do need to know size and make.