My setting has trolls that don't have a natural age limit. At least...unless they've ever been injured. Because their regeneration means that for every time they have to heal, a little more grows than should. So trolls get progressively more and more knobbly and distorted each time they regenerate, until they suffocate under the weight of their regrown flesh. And since this regeneration takes energy, trolls are always hungry. Which makes them get injured trying to acquire food (usually from something that'd rather not be eaten)...