A/N: If you know what dates I'm counting off of, then clearly you're either a Lit or history major and I salute you. I hope you enjoy this!
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He's only slightly older than Civilization. That being the reason he was created. He was younger than the first two humans, who gave up perfection for a less than holy existence.
He didn't see them be cast out. He didn't see the great War between his brothers and sisters. They rarely speak to him of it, as it is a sore spot in their history.
Though no angel is a child, he could be considered one in human terms.
He was built for Civilization, and that makes him singular from the scores of angels circling Heaven.
When Sam asks Castiel his age, he does not hesitate in telling him the exact number of years, months, weeks, days, hours, and minutes, going so far as to count the seconds if Dean hadn't stopped him.
He is nearing his six thousandth and first birthday. It was in his one thousand four hundred and ninety first birthday that he fostered human history. Each of his four thousand five hundred and nine birthdays since then has been great accomplishments for which he feels pride for (not too much, but one should like the work they do), memorizing important events in "history" housed within his years.
He would tell Dean about them, if he'd let him, but the boy only slaps down a bottle of not-so-cheap beer and tells the angel to drink up. Castiel drinks, and does not comment that he should be counting from one since his death; he feels like he's writing something new.