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Castiel always knew it was impossible for him to die of age. Of course as he lived his life around the Winchesters he'd grown so fond of, he realized he would be hard-pressed to die of unnatural causes either.
At least not permanently. Sam and Dean had this tendency to never die permanently, and no one they held dear to their hearts could break that unspoken rule either.
Castiel became one of those that was never granted permission to die permanently, even when he felt that he'd made mistakes that would damn him to endless nothingness.
Initially it was a formality, a brotherly promise between the three of them - Team Free Will - that stuck the idea that none of them would die forever. Then, slowly but surely, Dean changed the promise a little.
The promise began as unuttered words of, "You are family, and we protect family." Over time it shifted to fuller and spoken, "You are mine. And I protect what is mine." Of course those words were only uttered when Castiel lamented over all of his misdoings and tried to shut Dean out. Dean couldn't just say those things, it was against his code. He had to have a reason.
And if that wasn't one of the many things Castiel loved about him. His rigid necessity to follow rules only he knew. They were always the perfect match, a man that followed inescapable rules of his own creation, and an angel that broke every rule in the book.
Even Sam rooted for their love story, pulling Castiel aside to convince him to tell Dean how he felt, telling Dean to get the procerbial stick out of his ass. The younger hunter was not blind, and wished not only for his own happy ending, but a happy ending for his brother as well.
Castiel had wished that same fate for the three of them, regardless of what that meant for Cass and Dean specifically.
He had realized not long ago that Dean wouldn't last forever. Of course he'd known this before, but he'd really begun to understand it, for the first time.
+
He'd breathed a heavy sigh, sitting at the ever-used table in the bunker. His vessel was the same as it had been since the moment he'd met Dean in the barn. Without the soul of Jimmy, and the ever-present grace of an angel, it would not age a day.
Dean, however, who was sat just across from him scrolling through something on Sam's laptop, had notably aged. It wasn't just crow's feet anymore, nor was it rougher skin. It was greying hair, it was thinner breathes, it was age.
Castiel had felt it in every moment that he was around Dean, not that it made him love the man any less. If anything, it made his love swell ever more. Dean had not left Castiel's side since the night that he'd professed his love.
+
That night was in the Impala, a night like any other, after a rather terrifying hunt. They'd been hunting a vampire nest together, only to find that it was the centre of a web of nests. As they had originally believed it to be a simple hunt, Sam had not attended. It was just Dean and Castiel. Dean had been dealt a few potentially lethal blows by the end, Castiel bleeding harder than he had in a long while.
They'd decided eventually to run, sometimes winning meant living, not killing the monster. Castiel had been able to stave the lethality of Dean's injuries, but they were both pretty beat up.
So Dean, breathing heavy in the driver's seat of Baby, had clenched his steering wheel until his knuckles streamed white. "Cass," he'd said, "I wasn't ever gonna say this, but… in case I don't live forever," he stopped and chuckled, "Let me start this over."
Castiel had let him talk, shutting his mouth and letting Dean get what he needed out.
"I know I'm gonna die. Could've been tonight, could be tomorrow. But I gotta…" He'd stopped again, most likely thinking his next words over carefully, "I love you, man. More tha- more than a brother. You're more than that to me, y'know? More than family." That's where he'd left it. He left the sentence to air out in the car, he left it to spin in Castiel's head.
Then he'd turned the radio up, and let Castiel sit on the information for all of a month, cutting the angel off any time he spoke something even remotely like the conversation.
Even once they'd had the conversation, nothing changed. They both knew they had feelings for the other, and they'd experiemented with kissing. Hell, they even changed the rules to share a bed. Nothing substantial changed for a long while though.
In fact, it stayed relatively the same for about four years until Dean took Castiel to a somewhat fancy restaurant on a pier. It was their first 'date'. Then their relationship became real, and they feared less.
Now it was simple, they knew they loved eachother. They didn't profess it regularly, Castiel far moreso than Dean ever did. They knew it even when they didn't say it.
It was wonderful, Castiel had to admit. Being able to lean over and steal a kiss whenever he wished. Being able to wake up in bed next to his lover and mumbled about nonesense. Being able to feel Dean. It was all breathtaking, and everything Castiel had ever wanted since…
Well, since forever, it felt like.
Regardless, Dean was older now. And Castiel was stuck at the same point in his life that he'd always been. The same dark hair, the same body, the same visible age.
He'd watched humans fade away before, many civilians rise and fall. He'd seen the rise and fall of men he never considered would fall. He knew age. He knew time.
Yet it made it no easier to watch time weigh on Dean. His first true lover. Since the first grey hair on Dean's head, Castiel had shown his love much more vigorously. More 'I love you's, more gentle shows of affection, more kisses. He wanted Dean to know he loved him up until the very end.
And the end really did come so much sooner than Castiel thought. Time really felt like a flash, even if in the moment it felt like an age. Looking back on the many years with Dean felt like such a short period of time.
It didn't feel like long enough.
Yet he remembered so clearly when Dean Winchester, the man who could never stay down, collapsed after standing from his favoured chair to ready himself for bed.
He remembered Dean's shout; remembered rising out of his own seat and just not being able to catch Dean.
He remembered bringing Dean to bed and healing his broken wrist.
He remembered feeling the weakness of Dean's heart when he'd held Dean's forearm.
He would never forget Dean's smile and the soft, "I love you, Cass," he'd uttered before planting a kiss on Castiel's head.
He prayed to never forget holding Dean close, knowing this story was coming to a finish.
He could never forget falling asleep next to Dean and waking up next to a corpse.
He could never forgive himself for not being able to protect what was his. For not being able to heal what could not have been healed. For not finding the antidote to age; to time.
Castiel had been forbidden from Heaven for some time, for choosing the Winchesters over the 'Almighty'.
For years he'd tried to break in, to no avail. He tried to make deals with angels, with no point. By the end of a decade he'd made no headway.
It became increasingly obvious that once a human died to natural causes they were impossible to reanimate. Even then, Sam had taken it upon himself to give Dean a Hunter's burial.
Sam was coming to terms with his own age, and wanted Castiel to promise to leave it alone.
When in the end Castiel would be left alone. Forever.
Without a Heaven that would accept him. Without humans that would house him.
He'd really never considered Dean's mortality when he'd kissed him for the first time.
He'd never imagined this ending when he'd restarted the conversation of love.
When he chose man over the immortal.
When he'd damned himself to eternity alone.
Dean had never lied about the life of a Hunter. It ended ugly. He really didn't live forever.