Board Thread:General Discussion/@comment-24726378-20150102200745/@comment-10452902-20150601024426

First off, just because someone joined the Royal Guard does not mean they are no longer Shinigami, they are just not normal Shinigami which means they can easily be in the running. Secondly, there is contradictory information and a great lack of information for anyone to be fully accepted as the oldest Shinigami alive but from what we have, we can get close. For my third point, it is possible that the former first Kenpachi is as old as he is even though she didn't look it since shes been around since the birth of the Gotei 13 and didn't die until very recently. As we know little about the other thirteen original captains, it's hard to say if they would be in the running or not with her. Now including the Royal Guard, again, we know very little about them since they only appeared in this new arc but we do know that at least Ichibei and Nimaiya were around since before the Gotei 13 which definitely means they are in the running. But we don't for sure know how old the Gotei 13 is or how old the former captain commander was when he formed the Gotei 13.

But like I said, there is a lot of contradictory information about the Shinigami. In all likelyhood, the Soul Society wasn't called that until roughly around when the Gotei 13 was formed so it's much older than the Shinigami. As for your question about whether the Soul Society or Zanpact-to is older, Zanpact-to are born from the souls of a Shinigami companion so even if the despite the shells created by Nimaiya existing before the Zanpact-to as both it and the Shinigami are needed for a Zanpact-to to be born. I can't really say whether or not Nimaiya or Ichibe is older though it may be tied but it's possible that Ichibe is older as the one who names all things. He could have been the one who gave the Soul King that title despite being younger than the Soul King and could have been the one who named the Soul Society whatever it was called before it and named the Gotei 13, named the Shinigami the Shinigami and so forth.