User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-101.160.191.17-20120522085708/@comment-5110811-20120526021455

Lucifer being on that list of the five things the colt can't kill easily trumps what Death said. Besides, I don't think Death was lying. I think Death meant normal angels. His statement is broad. There is a clear delineation between Archangels and angels. The top archangels are a rare anomaly compared to everything else except God and Death. For instance, here is Cas's quote about Metatron, ..."he took down dictation when Creation was being formed."

This clearly outright says that Metatron is older than the Leviathans according to Cas. I think because of this and Lucifer being on that elite list of five things the colt can't kill in all of creation, Michael and Lucifer are highly likely older than the Leviathans. Since Metatron is.