A/N: I don't know why this ended up all angsty. I think I just have a tendency to torture characters I like, and Steve is so adorkable. My favorite characters from Avengers were Steve, Tony (duh, he's played by RDJ), and Loki (I also tend to sympathize with villains). For some reason I'm on an Avengers kick right now, and I was reading some fic that brought up Steve's super-healing powers and I thought of a random Doctor Who line about immortality that was something like "The only guarantee is that you'll end up alone". Which is terribly sad and therefore I needed to subject an awesome character to it. Also, this is kind of AU-ish; like, it ignores Civil War and stuff.
The Only Guarantee
It takes about a decade, but one morning while they are eating breakfast after a large battle, Steve Rogers looks up and realizes that the silver streaks in Tony's and Clint's hair are being illuminated by the sun streaming through the diner window. Bruce's brown curls look dark grey, and even Natasha is showing signs of age, faint wrinkles forming around the outer corners of her eyes and the edges of her lips.
Thor looks mostly the same as ever, though there is a certain maturity in his demeanor that wasn't there when they first assembled as the Avengers. But, Thor is a god, and therefore not subject to the human rules of aging.
Later, when Steve is alone in his room, he scrutinizes his reflection in the mirror, and immediately regrets it. He is just over a hundred years old but his hair is still wheat-gold, his skin is still perfectly smooth, and his body is still strong. He looks exactly the same as he did so many years ago, right after they injected him with that super-serum. He looks exactly the same as he did in the 1940s, but it is the 2020s now and the world has changed just as his friends are changing, and he is getting left behind. Again. He will look exactly the same even as his friends slowly wither into death, getting replaced by a younger generation of superheroes.
All of Tony's tech knowledge can't stop it. Natasha's agility, Clint's marksmanship, Bruce's rage... they can't slow it down, not by much. If they are lucky, they will go peacefully in their sleep, finally resting after decades of fighting. Steve knows he won't get that chance. Either he'll be taken down in battle, or he'll end it himself, except that the latter isn't really a viable option; he's Captain America, and it is his duty and his honor to defend his country until his heart stops beating. He tries not to think so far into the future, because it is absolutely terrifying. The only certainty about it is that he'll end up alone, lost in the ever-shifting world that looks nothing like the one he grew up in. No white picket fences here, no all-American apple pies, no golden retriever waiting for him on the doorstep. No Avengers. Even JARVIS will probably vanish at some point. They will disappear along with Bucky and Peggy and all the others, and his hands are shaking at the thought so he splashes cold water on his face and tears his gaze from the mirror.
He smiles at his friends when he sees them, jokes around with them as always, and as time passes he pretends he doesn't notice when Clint's aim starts becoming ever-so-slightly shaky, when Bruce starts forgetting small things, when Natasha starts having a bit more difficulty executing perfect roundhouse kicks. He runs a bit slower to accommodate their feebler gaits and focuses more on defending them so that they won't suffer broken bones that will take far too long to heal. He notices Thor doing the same. When Tony starts complaining about his whitening hair, Steve and Pepper laugh at him and call him "Grandpa" even though Pepper's own hair isn't exactly strawberry-blonde anymore and some part of Steve feels guilty for mocking Tony's distress. He feels Tony eyeing his own still blonde hair suspiciously; when he chances a glance at the billionaire he sees Tony's dark eyes are filled with murky sadness. He shakes his head in answer to the unasked question, and Tony turns away, pulling Pepper to his side.
Steve pivots to face the window, leaning his forehead on the cool glass as he surveys the New York skyline- already a very different skyline than the one he woke up to- and he suppresses a sigh. A shrill ringing pervades the room; Fury is calling Tony, but everyone ignores the call since the energy blasts suddenly levelling Central Park make it all too clear what the call is about. Steve pushes away from the window and runs to get his uniform, simultaneously pushing away his self-pity. No time for that now.
He has work to do.