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Steve didn't notice it immediately. In his defense, first there had been a world war going on. So he was happy if he had access to a reflective surface to help him shave. He never exactly devoted time to examining his reflection. Later, he had to fight other kinds of threats to humankind. Although in the new century he had quite a luxurious bathroom — courtesy of Tony — with enormous mirrors where he saw himself every time he went in even just to wash his hands or get a cup of water, he still never noticed.
People say you get used to what you see all the time so small changes simply don't really register. The days blended together, and Steve never cared to pay attention to wrinkles and gray hair, or the lack thereof.
The first time it crossed his mind was when Bucky came back.
“He… he looks exactly like when I last saw him in 1941,” Steve told Natasha. His hands were shaking, and he felt like the air was running out of the bunker where Hill took them. Nothing made sense anymore. Hydra lived on instead S.H.I.E.L.D., and two people he watched die were back to life.
Natasha took his hand and squeezed it. “My intel says he was put in cryo between missions. I'd imagine that would have the same effect the ice had on you.”
Steve figured that made sense, so he didn't give it much more thought.
Not noticing small changes when you see them all the time didn't only apply to Steve's own reflection. There was another person he saw all the time, or at least every day — unless it was one of those unfortunate ones when one of them was away for work — be that a one-man mission or a business meeting half across the world.
Tony.
They hit it off almost immediately after Steve moved into the tower. It was first a lot of yelling and fighting that ended in regular post-mission angry sex, but after a few months, they both realized their anger stemmed from fear of the other getting hurt because they both cared for each other.
Since then, Steve had seen Tony every night before going to sleep together, and every morning he got to wake up next to him. Yet, he never noticed.
Not even when years later he stepped up behind Tony fixing his bowtie in the huge full-scale mirror of their closet. They were about to head out to a Maria Stark Foundation gala, so Steve was also wearing a bowtie and a nicely tailored suit. His was black though, while Tony was in white. Looking at their reflection in the mirror, Steve was reminded of their wedding attire from a few years before. He stepped next to Tony and put his hand on his waist, wondering if his husband was having the same thoughts.
Tony looked at Steve’s face in the mirror at the gesture with a raised eyebrow, then laughed.
“I give it a few years and there will be headlines about me being an old geezer or you being after my inheritance!”
“Huh, what do you mean?” asked Steve, confused. That was definitely not the reaction he was hoping for.
Tony raised his eyebrow even higher and turned to Steve, a hand coming up to scratch his goatee, now sprinkled with gray, just like his hair. “I mean, how old were you again when we got together? Minus your years in the ice, of course. 28 maybe? Because well, you don't look a day older than that!” Tony turned back to look at his reflection again, furrowing his brows. “I was already pushing middle age then. A few years and I'll look like Hugh Hefner next to you.” He grimaced.
Steve hugged him from behind, resting his chin on Tony's shoulder he found his gaze in the mirror. “Don't be silly, there are literally articles coming out every other day about how you're aging like fine wine.”
“Yeah, and you were voted sexiest man alive for five years, even if not consecutively!” Tony rolled his eyes jokingly.
“As I recall, you beat me to it one of those years!” Steve laughed.
Tony turned around in Steve's embrace and wrapped his own arms around his husband's neck. “We are one fine couple, huh?” Tony smirked and kissed him.
Steve hated the idea of making Tony insecure with his youthful look, so he decided to grow a beard after that. As much as Tony teased him for it first, he ended up loving it. At first, Steve thought that solved the dilemma. However, as Tony's entire head of hair started to turn gray, Steve finally noticed that it was not his natural blond color hiding his gray hairs. He simply didn't have them.
Just like he didn't have wrinkles, either.
As the years passed, more and more new heroes joined the ranks of the Avengers. It had been so long since they joined that it already felt like Sam, Rhodey and Wanda had been with them from the beginning. Peter, or “the kid” as Tony still liked to call him, had grown into a responsible senior member. The overeager teenagers, Kamala, Kate, Cassie and America, who Steve tried so hard to hold back from playing heroes and getting hurt, were now in their thirties.
The first founding member to retire was Clint after he finally lost his hearing completely and it had started to affect his reaction time on the field. Next, it was Bruce whose weakening human form had been suffering more and more from the transformations.
But through it all, Steve remained the leader of the Avengers, both for the harsh training regiments and on the field missions. His stamina remained unchanged and unchallenged.
“I'm thinking about retiring, too,” Tony said one day, and it took Steve entirely off-guard.
“What? Why?” He looked at his husband, shocked.
“Because… my reflexes are getting slower. It won't be long before I’ll be more of a nuisance on the field than an asset. I don't want to put anyone on the team at risk,” Tony explained somberly. “You got Riri now anyway! Until she surpasses me in that field too, I will continue helping out with the gadgets though, don't worry.”
Steve had the hardest time getting used to not having Tony on the field anymore. He loved the team with all their new members, and he trusted them with his life, of course, he did. But none of them understood what he meant from half of a muffled word through the comms the way Tony always did.
It was hard, but not having to worry about Tony's safety was a nice change. Knowing Tony was waiting for him at home after every mission kept Steve motivated not to mess up on any of them while simultaneously finishing them as soon as possible. The sturdy suits Tony kept designing for him made sure he got home without a scratch for the serum to heal.
After a while, they were having sex less frequently. Not even Tony's playboy libido could have ever matched Steve's serum-enhanced stamina, but in the first decade and a half of their relationship, they'd never seemed to be able to get enough of each other. In recent years, lust had been replaced with cuddles. Even if Steve didn't want Tony any less, most days Tony was simply getting tired more easily and had fallen asleep by the time Steve got back from the shower.
However, there came the day when Tony simply couldn't.
“I’m so sorry,” Tony whimpered, voice muffled by the pillow he was burying his face into. “I knew this was gonna happen sooner or later, god damn it!” He bumped his fist on the mattress.
“Tony…” tried Steve, “It's normal. Nothing to be embarrassed about, really.”
“It's normal for a pathetic regular human like me. But you…” Tony suddenly lifted his head and turned to Steve “You should stop wasting your time on me. You should go find someone who can… match you.”
“I'm gonna pretend I didn't hear that!” Steve snorted. “You wouldn't match me? Tony, we literally understand each other without words, we always have. I'm half a person when you're not with me!”
“I won't be with you for much longer,” said Tony, voice dark.
“Don't be silly, Tony, you're half my age!” Steve winked, trying to deflate the tension.
“I may have matched you perfectly before, but I don't anymore.” Tony sighed, sadly. “You deserve more than having to nurse an old geezer like me.”
Steve shuffled closer to Tony, taking one of his hands in his. “Tony, ever since I woke up in this century, I only ever wanted you,” he said with an earnest smile. “You're the funniest, most brilliant and most heroic person I know who also happens to be devilishly handsome and mind-blowingly sexy. I can't see myself wanting anyone else.”
Something especially heartbroken flashed in Tony's eyes at that, but Steve didn't push it. Tony took a deep breath and then slowly nodded.
“Besides,” continued Steve with a shy smile, “please don't feel like I'm pushing it, but… I got some other ways to make you feel good if you're still interested.”
“Like hell I am!” Tony laughed, a little bitterly, but leaned in to kiss Steve.
It happened a few months after Tony had turned 80. Steve was sitting on the edge of their bed in the dark, uniform still on, his face buried in his hands when Tony found him.
“Hey, is everything alright?” Tony asked as the lights flickered on. “You haven't changed. Did something go wrong on the mission?”
Tony walked up to Steve and sat down next to him, placing one hand on his back and another on his knee. Steve shivered but didn't move.
“Talk to me, Steve. What's wrong?” Tony asked softly.
Steve finally removed his hands from his face, dropping them on his lap like they didn't even belong to him. They just lay there, palms facing up, while Steve stared ahead into nothing with glassy red eyes.
“Tony, I'm not aging,” he whispered, then winced as if the words hurt him.
Tony laughed in relief. “Oh, you aren't? Could have fooled me!”
Steve turned towards him and his pupils finally found focus just to glare at Tony, slightly offended that he made fun of his declaration.
“What?” Tony shrugged. “You thought I wouldn't notice you still looking just like when I first met you?” Tony smirked.
Steve opened his mouth and then closed it. He shook his head and drew his glance away from Tony to stare at his hands in his lap.
“I suppose it's a side effect of the serum,” started Tony. “Considering Bucky hasn't been aging, either. It must be healing tissue damage as it happens, not just injuries. Tissue damage is what leads to aging,” he explained.
Steve snorted bitterly. He only had himself to blame. He volunteered to undergo a procedure that had not been tested on anyone before. No one knew exactly what kind of side effects would be there. Steve wished the most annoying one was still his inability to get drunk.
“Hey, it's a good thing!” Tony nudged him with a smile. “You have so much more to do, to experience, you have so much more to live for. Plus the world needs you, Cap!” He winked.
“I…” Steve started but it ended up being swallowed in a sob. “I cannot live on without you…”
“You can and you will. You have a team to lead. There will always be people who need you.” Tony took one of Steve's hands and squeezed it encouragingly.
“I need you,” Steve sobbed.
Tony showed him a pained smile, then leaned in to kiss the side of Steve's head. “With some luck, I will be reborn and find you again in my next life. You gotta hold out for that too!”
Steve laughed bitterly. He turned to Tony and cupped his face between his hands, kissing him softly. “I love you so much,” Steve said as they parted.
“I love you too.” Tony smiled and pushed his forehead to Steve's. “I’ve been more than lucky to share this lifetime with you.”
They stayed like that for a little longer, then Tony turned around to search for something on his other side. From the crackling sound of it, he grabbed a plastic bag, then turned back to Steve.
“Blueberry?” Tony asked as if they didn't just have the saddest conversation of their entire relationship, then threw a handful of it in his own mouth.
Steve had to laugh, finally wiping away his tears. He had no idea if Tony realized he reenacted one of their first interactions from almost 40 years ago. Tony didn't have an eidetic memory like Steve did, but, well, he was a genius.
After that, Steve prayed every day to any and all gods he could think of to let Tony have a long life. But life had to be even more cruel to him. So, in the end, Tony didn't even make it to 90.
“He lived a nice, long life all things considered!” That was what everyone told Steve, but he could find no solace in it.
They were right though. Steve had almost lost Tony on missions more times than he liked to count because that stupid heroic man had to make the sacrifice play at every chance he got. And also, Tony's body had gone through more than most people's. The torture in Afghanistan, the shrapnels and the massive metal container of his arc reactor that had been inside his chest for years, the major surgery to remove them, a year of palladium poisoning, and countless other injuries he had collected on missions. The list went on and on. And that didn't even account for bad lifestyle habits like skipping entire nights of sleep on the regular and extensive consumption of hard alcohol and caffeine.
To be fair, Tony did cut back on alcohol and started to sleep more after he got together with Steve. “He had you to thank for his health, you were a good influence on him!” people also said. However, that only made Steve fall into the inescapable world of never-ending what-ifs. Did he do enough? Should he have pushed more to make Tony quit alcohol and caffeine entirely? To stop eating junk food entirely? To sleep at least eight hours every night?
He probably could have. But what kind of marriage would it have been if he treated Tony like a strict parent instead of the equal partners they were? And as Tony liked to point out in his last years, it wasn't like Steve couldn't have died first, on a mission, for instance, to be the one to leave Tony alone.
It was not what happened though.
Steve barely left their penthouse for months after Tony's funeral. Bucky came to see him every once in a while. He was the only person who could understand what Steve was going through, even if not to a full extent because Bucky never had a person like Tony in his life.
More and more funerals followed as slowly all the people Steve had become friends with in the new century — that was soon going to be the old century — disappeared from his side. Steve felt frozen in time — about which he was sure Tony would have had some good jokes to make. He didn't dare to make friends anymore, to get attached to people.
He only ever joined the Avengers for missions where he was absolutely irreplaceable, wearing the last uniform Tony designed for him, even as it started to turn into rags. He refused all and every new gear the resident geniuses offered him.
On every mission, a part of Steve hoped he wouldn't make it back alive. But every time he got in grave danger, he remembered Tony's voice making him swear he'd take care of himself and come back to him, and he couldn't possibly betray that promise. Not even when there was no Tony to return to anymore, because what if…
Steve had never believed in reincarnation. He doubted Tony had, either. But considering they had seen and experienced so many unlikely things in recent decades, from aliens to magic, Steve wasn't going to rule it out entirely. The world had gotten so weird, what was reincarnation compared to all that?
It was silly, but it helped Steve get up in the morning. To suit up before missions. Even so, it took him years before he accepted to lead the Avengers again. As always, Tony was right, helping people at least made Steve feel like there was some point to his unnaturally long life. That’s how he chose to call it at least, because he was unable to acknowledge it for what it probably was: immortality.
Steve loathed the serum for the longest time, but he needed to realize that without it he would have never even gotten to meet Tony. It was both a gift and a curse.
“With some luck, I will be reborn and find you again in my next life. You gotta hold out for that too!”