What are you wanting to do with it? Light riding at 2.5 or 3 but nothing heavy or too strenuous is my preference. That being said, I don't start mine until they're 4 and no heavy stuff until they're 5 or 6 but I'm a big gal and I'm paranoid
I don't believe in ridning any horse untill they are at least 3.5 years old. The growth plates in thier knees do not seal untill 3 years old and the spinal growth plates don't innish sealing untill they are 7., This is irrigardless of breed. You can do a lot of damage to unsealed joints by riding them
Some breeds can LOOK very mature but skelataly they are not!
Also you have to take into account the horses mental state. My boy wasnt ready untill jan this year (3.5years old), he would not have coped mentaly.
I believe in letting babies be babies, my lad has been in a field with the older boys and in a field with older mares, he is socialy well adjusted, been handled well. He respects me and my authority, he trusts me, but he is very much a pony with a character and a zest for life. He will be cheeky and push his luck, he is interested in what is going on around him, he does have that sparkle and I do put this all down to giving him time to grow up on his own and not doing too much too early!
Sailor, my 4 year old, is just now beginning to act like a mature horse. He's still silly and easily distracted but he CAN focus now some. I'm taking things slow. Like I said I'm a big gal and I want them to be sound forever, lol.
Besides the other reasons not to ride a horse too young (growth plates, fusing, etc) I read that a young horse is more apt to clamp its back muscles to protect its young back from too much weight, and therefore will learn to always clamp up when a rider gets on. I wish I could find the article again. :(