I ride a 3 year old, and he was trained to race, so he was broke to ride at an early age. When I first started riding him about 4 months ago we were working on, getting him used to my leg, and picking up the correct lead at the canter. That was the two main things he needed to work on.
Now he picks up the canter on the correct lead from the walk and trot, he bends around my leg at all three gaits, and is leg yielding very nicely. He also has picked up on voice commands, he knows, walk, whoa, stand, and two clucks means to canter. (I actually taught him voice commands by accident).
Now we are working on keeping an even pace at the canter, mainly, but also the trot. And we started doing ground polls and jumping small cross rails.
He is progressing very quick, he is a fast learner. But it does all depend on the horse...I know 3 and 4 year olds that aren't even saddle broke yet.