I think that 2 1/2 is a good age to start training. But that means I would have to break them to drive really quickly or them might get confused with their riding training, when they become 3 year olds. Im confused now????
I don't personally know ( I, myself don't do this) but when my brother breaks (he's a horse trainer) the Drafts and riding horses, he starts them at 2 1/2 or 3 years old (depending on growth) to ride and drive. He does both the training at the same time. (well ride one day drive the next or something around that) and he hasn't had any problems training them to do both :)
I,d agree with almagron,light driving wont do them any harm,its not saying pull a dray or anything but theres no weight in the light exercise carts I always keep haevy cobs and always start them at yearlings
The type of shafts wo,nt matter,but until your horse gets used to the cart wrap rags around the ends so if he turns sharp they wont dig his side make sure the shafts are fairly snug against her so if she kicks she is less likely to put her leg over,wen I put a horse in a cart the 1st day I tie it up allday attached to the cart and during the course of the day lift rock and push the cart against him so he gets used to the feel of everything especially the breeching hope this helps