My long deceased first pony was backed at 3 and jumping within weeks of being backed. I got him aged 8, he had been produced by a leading showjumper at that time (we are talking 1976 ish) and his then owners had high hopes for him. He got so sour so young he had been turned away for nearly 3 years before we got him ... for the huge sum of £200. He had back problems and behavioural issues, was really cold backed, reared, you name it. They got rid of him because of his issues and the fact that he would always have a fence in a jump off.
Not an ideal first pony but we had a lot of fun together but we gave up showjumping, he would always jump clear but when it came to a jump off was consistent with 4 faults - to this day I believe it was because he had been pushed too far too young and the knock down in a jump off was his way of telling me not to push him.
Bless him he lived til his late 20's, I never sold him but when I was about 18 or so he was semi retired to my uncle's racing yard and used to go out on the gallops nearly every day showing the fancy tb's how its done!
I now have an almost 3 year old Friesian colt. He was bitted in Holland at 18 months or so, and will be backed in May/June next year and then turned away again. My plan is to start him really slowly and carefully, allowing him to mature in his own time.