They have to leave the bunker.
The nice lady at the bakery shop Cas is so fond of is now 60 and she's bound to notice that while new lines are appearing on her face, Cas has not changed since they first met when she was just turning 48 and he looked 36.
People don't notice until they do, so they move.
They talk to Sam, and agree on staying in touch and on frequent visits, at least on Sam's part, and they leave. It's sudden and a bit sad, leaving the first real house they had, but they really have no choice. They are retired, now. Everything's calm and quiet apart from the occasional salt 'n burn. Heaven and Hell are as peaceful as they can be.
They move to the suburbs of a big city, big enough to go unnoticed, but calm enough to live safely. The neighbours welcome the couple with open arms. They friendly greet the man whose back does not seem to be so good anymore, but whose warm smile enlightens the room. They know his name's Dean and that he has been married for 10 years, which makes him 50, now. They know his husband's name is Castiel, and that he's just a bit younger than Dean.
When Dean turn 55 they all party together, cheering on him. They say age suits him, that he is like a fine wine. They compliment Cas, too. They seem amazed by how young he still looks, but they are not really surprised by it. Some people just don't seem to age, they say.
They greet them affectionately when they cross them on the street or returning from some shop, and sometimes they even stop to talk about everything and anything. People do not really know their story. They know it's tragic enough, though, so they don't ask. They are just happy to see them quietly laughing on their porch in the evening, Dean's shoulders covered by a soft blanket and Castiel's eyes smiling around their ever-present wrinkles. They don't see them kissing that much, but they often catch them holding hands. Sometimes hugging.
The day Dean turns 56, he's wearing glasses. He complains about them for a bit "I can't cook with them on, Cas, I can't see anything. Everything's fogged up!". But he soon gets over it.
They avidly listen to Cas's stories about exotic lands and distant times, not knowing he actually lived them. They don't need to know that. They also listen to him talking about how Dean sometimes falls asleep with his new glasses on because he's not really used to them yet and how he's getting grumpier about not having enough sleep but can't seem to hold off on coffee. He tells them it's a habit he made during the years he spent on the road. He also tells them how much he loves him. They don't need to know that, either, but he tells them anyway.
They move just after Dean turns 60. Again, people don't notice until they do, and Cas not looking a day older than when they first met him is starting to make people bewildered. Dean drives his faithful Impala until his knees and back hurt. Castiel doesn't mind taking the wheel, though. He knows how to take care of her as well as him.
They move to a quiet place, near a river, because they need to exorcise Purgatory's memories. Castiel still looks forty at most. Forty-two, if he keeps his brow furrowed. They do not have close neighbours, but they know some people from the near town. Some look at them strangely when they say they are married, they've been married for 20 years, but they don't really comment. It is a big enough age-gap, but they do not really care about it. They have seen worse things, they say.
Dean's hair is getting whiter and whiter, and he sometimes complains about it, but Castiel shushes him and kisses him and tells him he's loved anyway. He is always loved and more importantly, deserving of love. Dean still finds it hard to believe it, but sometimes he does, and Castiel's smile is the biggest reward of them all. Sometimes, Dean's joints hurt so bad he does not feel like moving from their bed. Castiel's tries to heal him through his grace, but his power is not as it used to be and sometimes, rarely though, it does not work. Dean's been fighting his whole life and he is bound to have these pains. Castiel lays with him, waiting for the painkillers to work and caresses his face, tracing the new-forming lines. He still counts his freckles when Dean's asleep.
Eventually, they have to move again. It is getting tiresome, because of Dean's age, but they would not change a thing. A thirty-year gap, though, becomes too much. Castiel still looks forty, and Dean is now seventy, though still young inside, as he claims. "And grumpy" Cas adds. They move again to an even quieter place, and they tell that they are uncle and nephew, to those who ask. They do not really meet many people anymore, though. They are happy as it is.
Of course, he and Cas have talked about after. But Cas just says not to worry about it. That they have a profound bond that has not been erased nor it will ever be.
They still meet with Sam, though. He is getting older, but he is happy, too. Happy and married and with grandchildren on the way. And he has a dog, too. The one Sammy's always wanted and now can afford to have. He still has long hair and Dean still complains about it. "If I had a pair of scissors near me, Sammy". And "Your sight is getting worse, Samantha, wouldn't want you to fall because you had hair in your eyes again". Castiel still laughs about it and at them.
They have to move again. People believe them to be some kind of close relatives, now. Some think they are grandfather and grandchild, even if they show no resemblance. They do not really see Dean a lot anymore, not since he turned 91 and he likes to stay in the calm peace of his house. Sometimes, when the sun is high and Dean's bones allow, they take a walk and hold hands again.
Their smiles have not changed since they were both young –well, since Dean was. Dean's hands are wrinkled and bony, but they are still warm as ever. His hair is completely white, now, but "Hey, Cas, at least I didn't go bald" to which Cas answers "Of course, Dean". Not that to Cas it matters, anyway. Dean's soul is still bright, still scarred, but somewhat pure.
It continues to be even when he takes his last breath after his 94th birthday.
Castiel goes back to the bunker. He tours the near city for a bit, passes by the nice lady's bakery shop and sees a young girl working behind the counter. He knows she is her grandchild and he knows the nice lady has an even nicer Heaven, now.
He goes back to Sam and does not say goodbye, he says "I'll see you later, Sam".
Near the bunker, right behind the trees, a shape of wings decorates the ground.
People don't really notice, until they do. And when they notice the shape of wings, partly covered by moss and grass and fallen branches, they start talking about it. They invent legends and talk about that strange man, the one who always wore a beige trench coat and knew ancient tales as if he really experienced them. The one their grandparents told them about and the one they have seen themselves exactly as they described him. They ask Sam about it, sometimes, but Sam just shakes it off, blaming his memory. He knows they would not rationally accept the truth, and he knows that they really don't need to know. Castiel and Dean's love story is tragic enough and legendary on its own. It's epic, even.
In Heaven, Dean is sitting in his room, which is strikingly similar to the one he had in the bunker. He is animatedly talking with Charlie and he does not look a year older than thirty now.
Some footsteps echo in the hallway and suddenly, the door opens and Castiel's warm and happy smile appears behind it. Dean launches himself at him.
"Hello, Dean". He hugs him tighter.
Dean has never looked younger not happier.