I was thinking about how kiado could be immortal and I may have figured it out. So far oda has explained the ope ope no mi and its power, the perennial youth operation. I don't understand why oda would explain that if it will never come into play. Obviously law isn't going to use it on anyone of the straw hats because if luffy was immortal it would kind of kill the suspense in fights. But what if the former user of the ope ope no mi used it on kiado and that's why he is immortal. Kiado looks like he's in his 40's and law in his 20's so the time would match up. Would make for an interesting story