Chandab is right. It is nearly imposable to start training too early. Being ready to drive when physically able involves early, comprehensive and above all fun, training as early as possible.
My favorite quote that I was blessed to be present for was when a wonderful Lady kept exclaiming to her husband "Look dear she's trotting SHE'S TROTTING. Now she wasn't real young but I guess that trotting was something she always had to be pressed to do with standard rigs. Making everything fun and even playing with the training will put you in a great position to start when the animal is physically ready. And best of all the bond is strengthened and more trust will be in place when the unexpected is come upon OR asked for by you. If the bond is everything, and it is with these guys, anything that strengthens the bond should be sought after.