Neither of them say it out loud until there’s a period of three weeks when Tony ‘dies’ and Steve ends up in a coma from throwing himself into the path of a raging killer robot.
He wakes up days later with Tony glaring at him, and his smile is sluggish. “Oh,” he says, soft. “Thank god.”
"I’m alive, asshole," Tony snaps. "We both are, no thanks to you, god, Steve-” and then the nurse comes in and Tony falls silent.
It takes several days before Tony takes a deep breath and says, “So, you were serious when you said you didn’t want to go on without me.”
Steve’s fingers pause in where they’re undoing his shoelaces. “Tony.”
"Don’t ‘Tony’ me," Tony says, and has to close his eyes for a moment. "Steve, I’m- I’m sixty. I’m not going to-"
"We have a long time left," Steve cuts him off, eyes suddenly bright. "We have decades, Tony, don’t-"
"Steve," Tony says, and Steve falls silent. "Fuck, Steve- you can’t-"
He pushes his hands through his hair, fingers working through it- his hair has more streaks of grey every time Steve looks at it, his face more lined, and it’s starting to make Steve panic every time he notices a new wrinkle in Tony’s skin.
"You’re going to have to keep going without me."
Steve knows this; has known it ever since he took a test and they announced that he’s going to be 400 by the time his body acts like he’s seventy years old.
Even so, Steve can’t stop his jaw from locking, his throat closing up as he looks over at Tony. “Says who?”
"Says-" Tony sighs, loud and frustrated. "Everyone, Steve, says me.”
"You could invent something-"
"That never turns out well, you know that," Tony says quietly. He takes Steve’s hands in his, and Steve all but clutches them. "Steve. You’ll get over it, one day."
"Don’t you dare," Steve says, just as soft. "Don’t you even suggest that."
"I know it doesn’t seem like it now-"
"You’ll invent something," Steve says. "You’ll make something, and you’ll stay with me. Or you’ll- you’ll get hit by something which makes you younger, or something will happen- something always happens, we’re superheroes, people don’t- they don’t stay dead, Tony, you can’t-"
He chokes up then, voice breaking, and Tony rests his forehead on Steve’s shoulder as it shakes.
"Don’t leave me alone," Steve says, voice hardly recognizable with the sobs wrecking it.
Tony doesn’t promise, because he knows he can’t. “We have decades,” he says instead, and holds Steve as he shakes apart in his arms.