When Steve returned to the 1940s to reunite with Peggy Carter, fans were left confused by more than just his abandonment of his friends. Aside from his out-of-character decision to leave the place he called home at the end of Avengers: Endgame, Steve’s unwillingness to tell Sam where he went and what he did in the past created some major plot holes in the MCU. The inconsistencies can be traced all the way back to Captain America: The First Avenger, when Steve received the “perfected” supersoldier serum during World War II.
Bucky received a more watered-down version of the serum from Hydra, since Dr. Arnim Zola didn’t have the recipe for the same serum Steve was injected with. Despite the flawed serum design, Bucky and Steve are matched in physical strength, agility, and endurance, meaning it’s safe to assume that the different serums are quite similar in design. This also means that Steve and Bucky have extremely similar, if not identical, biological and genetic capabilities.
Bucky has been alive and kicking for over a hundred years, with only brief periods of freezing when Hydra didn’t need him for any immediate tasks. Overall, though, Bucky has spent significantly less time in the ice than Steve did. If Bucky spent a total of 10 years on ice, he should look about 90 years old in the MCU films. Having harbored so much guilt and resentment for himself in The Falcon and the Winter Soldier, there’s no way that Bucky spent more than a decade in a restful state, because he’s done so much damage in the past as The Winter Soldier.
Between the years of training, the amount of time it took for Bucky to successfully complete his missions, and any sort of technological advancements Hydra had to make to his metal arm, Bucky spent quite a lot of time alive in the world. His brief cryostasis periods obviously slowed his aging down, but the fact that Bucky is still a man in his 30s after a century proves that supersoldiers age significantly slower than the average human, even with the periods of freezing.
Marvel Comics also can confirm that supersoldiers age differently than regular people, because their healing abilities are more advanced. Supersoldier’s bodies naturally cure injuries that would hospitalize others, meaning life-threatening illnesses are nothing more than a common cold to someone who has been injected with the serum. The MCU films haven’t gone into as much detail about the aging or healing process of a supersoldier, but Bucky’s youthful state in recent films after a lifetime of being The Winter Soldier proves that those with the serum take quite a long time to look death in the face
This brings everything back to Steve’s time travel adventures in Avengers: Endgame. Based on the context clues given to fans during multiple MCU films, Steve should be aging just as slowly, or perhaps even slower, than Bucky since he received a more enhanced version of the serum. If Steve can outlive the regular person, this means that he still spent years in a world without Peggy Carter, despite abandoning everything just to be with her. When Steve returns to the present day, he’s become an old man bearing no resemblance to the person he was upon making his exit.
Steve traveled back in time with the serum still in his veins, and when fans saw him dancing with Peggy in the 40s, he had the same physical form given to him by the serum. Time travel did not affect him biologically, and he was still a supersoldier when he found Peggy again in the past. This begs the question of what Steve did after Peggy had died. Assuming that the abstract aging process of a supersoldier causes them to outlive others, this means that Steve was alone in the 40s for years after Peggy was gone. In order to return as an old man, Steve had to have been there for quite some time before reaching a point where time finally caught up to him and he started showing his age.
As previously mentioned, Steve was also incredibly unwilling to tell his best friends what he did in his absence. So, fans are left without all the answers, as well. Sure, he saw Peggy and lived a life with her, but what happened when she was gone, and Steve still had years to go before becoming an old man? Did he travel to a time where Bucky was never captured by Hydra and never became The Winter Soldier? Did he know that Bucky was still out there being tortured by Hydra, but chose to do nothing about it? Did he eventually save Bucky, after Peggy had died and the two supersoldiers were left without any family or loved ones?
Overall, Steve’s decision left fans and even MCU writers both with a lot of questions. As of right now, there’s no indication that Marvel has plans to offer an explanation, and it seems as if fans are supposed to accept Steve’s choice and move on. It’s rare for Marvel to introduce such an anomaly without expanding upon it any further, but Steve’s reunion with Peggy is the most baffling of all.