I've started mine at two - ground driving - sometimes earlier just for fun because it is a way to interact with them as babies - but actually ground driving with a purpose? Two - and I've hitched them to light easy entry as two yr olds to pull around the yard and at three, they had it down and I could start actually going somewhere, adding a passenger (besides myself), working on flat surfaces with some long slow distance (lots of walking for the first few months and slow gradual trotting) and some horses took to it so well we were already going to shows as three year olds. I'd say two years is a great place to start 'conservatively' but there are variables such as temperament, size, development, etc. Each is an individual.
I agree with clipperdeclop for the education in long line (reining) and lunging and building up the horses education.
I normaly break mine at the age of four years old as all there bones and joints have developed in the horse.
Up untill then he has not stood idle he has been schooled on the lunge walked out on the road longlineing and pulling a car tire as well so his education has provided him with the feeling of his harness and the noises of a tire being dragged so with little or no fuss he can go straight in to the shafts and long lined beside him with the cart and on the next couple of lessons hop in gentley and drive a shought distance call it a day and let the horse think about it and drive him in the cart after that and build up in walk untill your horse is fit and you can stay out longer and ferther his education.
I started mine at 14 months pulling an inner tube and then a tire. When he was eighteen months I took him to the woods and pulled six inch diameter firewood logs to the truck with him. When He was old enough to hook to a cart he was solid as a rock.