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.
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:
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.
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.
In 2012 New York, it is established that the suits are time machines themselves: Tony and Steve can use their suits to hop other places than they were originally sent. There, they went to 1970 New Jersey, because they knew they could find both the tesseract and more Pym particles (since they were out of fuel).
Steve was expected to do exactly that on his "return the stones"* mission. He was given plenty of fuel and was expected to hop all over spacetime returning the Infinity Stones. When he was done, he could easily make one more hop to ~1947 America and go look up Peggy Carter and have a life with her. He could do that for 10, 20, 30 years, birth and raise Sharon Carter for instance, and when he and Peggy agree, he can then make an additional hop to 2019 to create closure with the Avengers before he passes of old age.
Since he can hop everywhere and everywhen else, presumably he can also hop to 2019 without needing the benefit of Banner's portal. And maybe that's what he did.
I don't think he joined Peggy in the late 50's/60s because that would leave her waiting too long. So I think he went to the 1940s. I also don't think he just "waited out" 1947-2019 because that's 72 additional years on his mid-30s? age, and that's 110 years old. Of course I'm not an elderly supersoldier, so what do I know :)
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.
Captain America got older because he spent most of the rest of his life in an alternate universe / reality, then jumped back to the main / prime universe (just seconds after he left the main universe). He did not grow old in the main universe.
According to the Russo brothers, from a Q & A (emphasis mine):
Q: Did Captain America's action at the end affect the timeline? Does that mean there was a time where two CA existed in a same universe?
A: To me, CA's action in the end wasn't the fact he wanted to change anything, it's more like me has made a choice. He chose to go back to past and lived with the one he loved for the rest of his life. The time travel in this movie created an alternate reality. He lived a completely different life in that world. We don't know how exactly his life turned out, but I'd like to believe he still helped many others when they were needed in that world. Yes, there were two CA in that reality, it's just like what Hulk said, what happened in the past has already happened. If you go back to past, you simply created a new reality. The characters in this movie created new timeline when they went back to the past, but it had no effect to the prime universe. What happened in the past 22 movies was still canon.
Q: EG's plot, is it a parallel universe or a closed time loop?
A: Nope, not a time loop. Both Ancient One and Hulk were right. You can't change the future by simply going back to past. But it's possible to create a different alternate future. It's not butterfly effect. Every decision you made in the past could potentially create a new timeline. For example, the old Cap at the end movie, he lived his married life in a different universe from the main one. He had to make another jump back to the main universe at the end to give the shield to Sam.
About his powers, it is not shown whether he still has it or not. I found no reason to believe that he lost his powers, though.
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.
It can be like..... Captain went to the battle between the young steve and red skull where the cap from future helps the cap in past that he wont go down to ice. Here the cap from past is not knowing the future cap and he is helping though. Then future cap takes the shield and moves to future gets old and comes back to reality. While the past cap is living his peaceful life with peggy and gets his dance. (the cap dancing with peggy can be the past cap.). This scene should be showed us to confirm that cap got shield, past cap survived plane crash.. Etc.. Otherwise if cap is living in past and coming throughout his life to meet bucky, sam and bruce to give shield.... There would be 2 steve(they will get the past cap from ice.). I think Russo brothers will give a correct explanation 😇
The ending leaves a gap on how Rogers returned to his own timestream. But here is what we do know.
He left through the time portal with enough Pym particles to make several jumps through time and space in order to return all the stones and return to his own time/dimension.
The platform serves as a launch pad but also as a retrieval device. Like a time and dimensional tug rope. Remember how each time you influence the past you create a new timestream So once you do this you would likely find it easier to jump back to your own timestream using the platform. The bracelets act as GPS for the platform to pull you back.
If Steve Roger's went back in time and stayed there he would grow old in a different dimension. He probably turned his GPS off or otherwise avoided being retrieved by the portal.
Now how he did it I'm not certain but we can piece it together.
They didn't make clear how the bracelets work. Did every avenger create their own temporal dimension when they jumped? Can the bracelets navigate between dimensions and not just time and space? It is never spelled out. But the ending implies this non verbally, and with some deductive reasoning, we can conclude a few more things. Because how else would Cap reach all locations in their separate timelines to return their respective stones. 1940s, 2014, etc... This would give the spiderverse a headache.
But let's say their previous GPS/time travel bracelets stored past coordinates so they could return there, to the correct time/space/dimension. This would explain how Steve Rogers was able to live out his life in alternate earth 1940 and then return on his own without going through the portal when he was old. He would return right where he left (give or take a few meters for dramatic effect).
Could other things have happened, possibly? He didn't want to say after all. But without outside factors, this is the easiest to explain his sudden reappearance without breaking their own rules for time/dimensional travel.