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In the end of Avengers: Endgame, why does Captain America get older when he returns from the quantum machine?

Also does this mean he lost his power too?

Captain America didn't return through the Quantum machine, he went to the past and lived his life. He got to the same area by just traveling there at the appropriate time. He chose to stay in the past and live a full life. That's why he is now older in the scene and he just appears on the bench.

There is no indication if he has lost any of his powers, we just know he has handed his legacy onto the next Captain America.

The first thought explaination is Draken's, but it is not possible, as going back in time cannot change the present, as said in the movie begining. Let me explain.

When sending Cap' back, whoever maneuvered the machine (can't recall who it was) answered the question "How long will it take?" by "For us, 5 seconds. For him, all the time he needs.". This statement implies that, no matter how long Cap had spent in the past, he would come back in 5 seconds through the time machine if he chose to come back. What happens next shows that he never came back through the machine.

However, when travelling through the quandtum machine, you don't travel through time, you change realities, like you were switching between worlds. The only way for Cap' to come back from this alternate reality he was sent in was through the quantum machine, which he didn't use. Therefore, he could not be there.

There is, however, a way that their encounter with old Cap' is possible. Old Cap' comes from another reality, from which he came to put back the infinity stones, and didn't return in order to grow old there. However, we know that this isn't the case, as Cap' met Peggy back in Winter Soldier, and we clearly understood what I just described did not happen.

This incoherence stays a mystery for me. The only possible explaination I have come to is that Cap' found a way to come back in the reality we know, after living his peaceful life in the one he was sent to. But in any case, old Cap' cannot have grown old in this reality.

I had this same conversation with a colleague this morning, and agree with TheWildHealer - This Steve Rogers, while being our MCU/Earth-199999 Steve Rogers:

1. Travelled to the past in a different reality, returned the Infinity Stones to their rightful places, and then dropped into the late 40s/Early 50s to be with that reality's Peggy Carter.


2. Chose to return to our Earth-199999 at that point in time to pass on the shield.

How is a different matter.

That said, being with Peggy would put him in with SHIELD, and from there he could easily have got in contact with the three required people to create a Quantum Time Tunnel - Hank Pym, Tony Stark and Bruce Banner - from his new reality to send him back to that point in time on Earth-199999 to pass on the Shield to Sam, and that he already has the combined Pym/Stark/Banner-tech suit for them to reverse engineer (which also gives them the temporal GPS location, as it would be the return location from the suit he wears when he leaves). We don't see what happens to him afterwards, so it is possible he turns up, passes on the shield, and then leaves again to return to the same reality where he has spent the past 70-odd years.

Equally, I can't see Bruce et al doing that Quantum Time Tunnel anywhere public - I would suggest that it is either on the grounds of another Avengers Facility, or in the grounds of Tony's house in the woods - either way, I don't believe it would be somewhere where the public are just going to be able to walk up and get that close to them doing strange quantum tunnelling things, so Steve's sudden appearance there would make sense if he arrived by another Quantum Time Tunnel.

This is my opinion, but it does fit the facts as we know them from the film.

My interpretation of this scene was that in our timeline, Captain America was always meant to end up with Peggy. When he goes back in time to put the stones back in their rightful place he doesn't stay in the splintered timeline, he returns all the stones and then uses his last Pym Particle not to return to where he entered, but to go back to some time after the events of Captain America: The First Avenger (and possibly after the events of Agent Carter depending on whether or not that is to be retconned).

So our version of Steve Rodgers, fought in WWII, was frozen for ~70 years, fought Thanos, lost and five years later, fought him again and won, after winning returned all the Infinity Stones to their rightful timelines, and then ended up with Peggy, grew old, then passed the shield to Sam.

I think the best explanation is the Cap on the bench has been around the entire time and is from a different reality. He was always the husband Peggy talked about. He stayed hidden and told Peggy not to tell the young version anything because it would ruin the timeline. It fits in the End Game time travel theory.

Timeline 1 - Thanos wins. Kills half, time travel happens, Avengers win, Cap goes back in time, he never shows up on that bench.

Timeline 2/Our Timeline - Cap goes back in time and returns the stones, he marries Peggy and stays hidden, All of the marvel movies we saw after Cap 1 happen, Thanos wins, time travel happens, Avengers win, Cap goes back in time, Hiding Cap is on the bench.