In the Marvel Cinematic Universe (MCU) film Avengers: Endgame, Captain America (played by Chris Evans) gets old due to his use of a time machine to return the Infinity Stones to their rightful places in the timeline. This action leads to him staying in the past and living a full life with his long-time love interest, Peggy Carter (played by Hayley Atwell).
To fully understand why Captain America got old in Endgame, it’s important to first understand the events leading up to it. In the previous film, Avengers: Infinity War, the villain Thanos (played by Josh Brolin) collects all six Infinity Stones and uses them to wipe out half of all life in the universe. In Endgame, the remaining Avengers team up to try and reverse the damage done by Thanos by traveling through time and collecting the stones before he does.
During this time-traveling mission, Captain America is given the task of returning the stones to their proper places in the timeline in order to avoid creating alternate realities. However, when it comes time for him to return to the present, he doesn’t show up. Instead, an old man is seen sitting on a nearby bench, and it’s revealed that this is actually an aged Captain America.
So, how did Captain America get old? The answer lies in his decision to stay in the past with Peggy Carter. Peggy was a love interest of Steve Rogers/Captain America since the first Captain America movie, and she was also a founding member of the Strategic Scientific Reserve (SSR), a precursor to S.H.I.E.L.D. In the MCU, Peggy is shown to have died of old age, but Endgame presents a unique opportunity for Steve to change that.
In the final act of the movie, after the Avengers have defeated Thanos and his army, Captain America volunteers to return the stones to their proper places in the timeline. However, instead of returning to the present immediately after completing his mission, he decides to stay in the past and live a full life with Peggy.
It’s important to note that the concept of time travel in the MCU operates on the idea of alternate realities. This means that when someone travels back in time and changes something, it creates a new reality that branches off from the original timeline. In order to avoid creating more alternate realities, the Avengers are tasked with returning the stones to their original places in the timeline.
However, Captain America’s decision to stay in the past with Peggy seems to contradict this rule. If he stayed in the past, wouldn’t that create a new alternate reality? It’s possible that he did create an alternate reality, but he may have also found a way to return to the original timeline without causing any disruptions. This is left open to interpretation and is not fully explained in the film.
In any case, Captain America’s decision to stay in the past allows him to live a full life with Peggy and grow old with her. When we see him at the end of the movie, he is an old man who has passed on his iconic shield to his friend and fellow Avenger, Sam Wilson (played by Anthony Mackie).
In conclusion, Captain America got old in Endgame because he chose to stay in the past and live a full life with Peggy Carter. This decision may have created an alternate reality, but it’s not fully explained in the film. Regardless, it allowed him to pass on the mantle of Captain America to Sam Wilson and retire as an old man, finally getting the chance to live the life he deserved after years of fighting for others.