Thing is, a fast metabolism tends to kill you through wear and tear on the cardio-vascular system. Since you do have a quicker cellular healing rater, your more likely to die suddenly than suffer afflictions from aging.
Mind you, in the world of dominions it doesn't have to be a fast metabolism providing the healing. Maybe the wizard has fell into a healing spring, or discovered the fabled philosopher's stone. He could have had some kind of magical skin graft or blood transfusion from a naturally regenerating creature. It could even be due to the favour of his God
