Yeah, we're pretty amazed at how big Cody has gotten! I've heard he could continue to grow until he's 7 but he's looking pretty mature to me, although he rarely went through an awkward phase. I started him doing ground work and we ponied him on trail rides a lot. Then when he was 2, we saddled him up and would pony him and once in a while after 2 1/2 put one of my daughters (50-70 lbs) up on him (it was never a big deal having a rider on him) and finally when he turned 3, I got on and would walk him around some. At 3 1/2 I started riding him on short trail rides and gradually increased to trotting. We didn't canter until he turned 5 and started jumping him a little recently. He's still pretty green when it comes to arena work but he's exceptional on the trail, he's been everywhere but the ocean!
I fed him Triple Crown Jr and he did very well on that, didn't want him to grow too fast and end up with joint problems.
Keep us updated on your guy's growth! With Cody, I did the height predictor of measuring from the dent in the front of the knee to the coronet band. That length in inches is supposed to be the equivalent of hands in the mature horse. Cody measured 18" and is basically 18 hands. I think you do it at 2 yrs. old.