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!