@Regenbogen:
Upon what do you base that? Wizards still age… just more slowly. It makes more sense for the “good copy” cell replacement to break down over time for wizards as they age… since they do age and will eventually die of old age… just a lot later than normals. Consider River Shoulder’s worries about Listens to Wind because of LTW’s age…
Yeah, probably with extremely old age the process of cell renewal would be slowed. But we are talking about a wizard in this prime.
My theory: wizards age more slowly than vanilla humans. According to Butters, Harry's cells renewed themselves to more exact copies than usual. This means that the healing process of severe injuries appears quicker compared to vanilla humans, because a vanilla human doesn't heal completely.
Take severe burn scars for example: they never go away completely and patients often loose at least part of the function in the burned areas, which never heals completely. So they are never again near as healthy as they were before. Harry by now has full functionality in his burned hand. So severely burned that a doctor tried to convince him to let it amputate. If he wasn't a wizard, his hand would still lack a lot of functionality.
Wizard in his prime:
According to the impression I have from the books, young wizards seem to age at the same speed as vanillas up to let's guess their fifties or sixties. When they reached that age, they seem to age more slowly than before and stay in this phase of appearing 50/60ish for centuries until they slowly reach the 80/90 years and show signs of old age. I guess this is as individual as with vanilla mortals. Some age faster and die sooner than others. Only with wizards, the difference is counted in centuries not decades.
So a wizard in his centuries lasting 50s or 60s (one of those, I guess, Kemmler was) who ages more slowly than at a younger age, will therefore heal faster than a younger wizard, because his cells regenerate faster. Once he has reached a wizards old age after maybe 5 to 7 centuries, the healing speed will decline.