I start riding my horses when my vet says their growth plates are fused, which definitely varies among breeds as well as the individual horses. For instance, my paint mare was cleared at 2yo. My racebred qh who had epiphysitis as a yearling when I got him, was 3yo before cleared for light riding, and my perch/arab was almost 5yo before he got full clearance.
However, between ponying them on trails, getting them used to wearing the saddle, hanging out on top of their back and doing little stuff at the walk, all of which I do from yearling on, when it comes time to ride them, they feel already well started.
I live in the northern mountains of Utah with my wonderful husband, 5 horses, 4 dogs, 2 cats, 32 geese and 9 ducks. Life is good.