We once took a nine-month old colt to the Midwest Horse Fair for the mini demo. My teenage sister handled him in a halter obstacle demonstration, walking through hoops, over tarps, stuff thrown on him. He was SOOOO calm with the noisy Coliseum, the crowds, the music, etc. that we could have sold him multiple times that weekend!
I think Mom sold him before he was broke to drive, but it's that type of handling at that young age that will do them a world of good.