Depending on the breed I don't get near my horses until they are 3 and a half. I know my mare because of her breeding I wont touch or do anything until she is at least 4. She's going to be growing for quite a while.
When I do start them its all ground work for the first few months or the first year, give their joints time to heal. The last thing I want to be doing is injecting their hocks because they've been over worked.