So, Grace was not only exposed to the atmosphere of Adrian (not directly, but we don't know that that space suit kept everything out, this is a completely alien world after all) he also exposed himself to astrophage, and ATE tauomeba. Tauomeba which is endlessly adaptable to different environments. So, who's to say that while Grace is on the way to Erid his body isn't being changed by the Tauomeba. What's to say that after he gets to Erid and has been living there a while the Tauomeba doesn't change his body even more.
Grace isn't looking for it, because he was just trying to survive long enough to find the cure for astrophage, then to survive long enough to get Rocky to Erid and help fix their astrophage problem, and maybe even find a way to survive there himself. So, it's a surprise to him when he realizes that Erid's gravity has stopped having such a strong effect on him. He rationalizes it away as just getting used to the new environment (he knows it's not that), and he explains his lack of gray hair away by insisting that maybe his parents didn't go gray early. (they did).
Grace can't lie to himself forever, and when he does the math and realizes that he turned 60 but he still looks like he did when Rocky met him he almost doesn't recognize himself. It's him, just as young looking as when Stratt sent him into space. How can this be?
Grace doesn't tell Rocky at first. His friend knows something's wrong, but Grace and Rocky have known each other long enough to know when to poke for answers and when to wait. It takes Grace months of ruthlessly running tests on his own biology and physiology to come to the conclusion that the Tauomeba has made his body just as adaptable as it is, and that one day Grace himself may even be able to tolerate the full Gravity of Erid without the Habitat that his hosts have so generously created for him. He may be able to interact with his chosen home without any walls between them.
That's the day Grace decides to tell his best friend what's going on. He's not aging the normal ways that humans age, and his body is becoming more resilient than a human body was ever meant to become. Rocky, who has been begging the scientists who have been working with Grace for a way to keep his best friend around for as long as possible has never been happier.
*This is very handwavy of any and all scientific principles because I was educated by the American Public School system. Bless. *
