An average life span for a horse is around 20 -25 years, though they can live for up to 30 years. The oldest recorded horse was "Old Billy," an English barge horse, who lived to be 62 years old.
as with dogs, larger horses have a smaller life span than the smaller ponies. there is a shire hore living in west cornwall that is still going at 40 though which is quite incredible for an horse of this size
One of my friends had a Shire who was still working when he died at 26. He kept trying to retire him, but each time he did, the horse would fret and make himself ill when he saw the others going off to work.