It depends on their health and energy (if the horse doesn't have Achilles Heel). Before 25, a horse can lose health if you forget to groom them, they are overweight or underweight, or you bed them with less than 20% energy. Horses over 25 without a P. Stone lose a few % of health with each aging, but improper care or overworking causes it to drop faster. Horses with P. Stone cannot die.