User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-5234995-20120626161614/@comment-5110811-20120707155514

There's already a small contradiction to Death's statement when Cas said Metatron wrote down dictation when creation was being formed by God. Which means he was around before the creation of the leviathans. This leaves Death's broad statement to a few open interpretations. Which means there is a high possibility that Michael and Lucifer are older than the Leviathans as well. Michael, Metatron, and Lucifer are very likely primordial angels.

Don't know about Raphael and Gabriel though.