Realistically, if Steve didn't decide to retire and make Tony do it with him, Tony would keep going until he died (via supervillain, probably, rather than via age). Getting older wouldn't affect his abilty to do the whole superhero thing as much as it would other heroes, because he could have the armor compensate. I mean, the armor was compensating for Tony not being able to walk at one point, and being sixty and having bad knees is a lot less of a physical hurdle than paralysis. He could just keep upgrading until he'd essentially turned himself into Metallo sans kryptonite, or a Borg.

So, probably, once Danielle "Power Fist" Cage-Jones and the others have confront Steve about why they think he and Tony ought to retire, Steve's going to have to, like you said, convince Tony to do it for his (Steve's) sake.