deans1911:

So in The Man Who Would Be King, Castiel says that he’s been around for a while. In his opening monologue, the oldest memory he presents is the origins of terrestrial life from the oceans. Let’s start with that.

Paleontologists believe that prehistoric fish first evolved into tetrapods around 390 million years ago, so Castiel is at least that old. Now, when he rebelled at the end of Season 4, he’d known Dean for about a year:

1/390,000,000 = 0.00000000256% of Cas’s life

If you condensed all of Castiel’s known existence into a 2 hour movie, Dean wouldn’t show up until the last fraction of a second of the ending credits. But Cas still rebelled for Dean.

That’s some powerful shit, and it’s canon.