At the end of a long life
Old Man Steve is none other than the version of Steve Rogers (Captain America) that we see at the end of the movie Avengers Endgame. After Thanos wipes out half the population with the Infinity Stones, the surviving Avengers try as best they can to get back to normal life. But the return of Scott Lang proves to them that there may be a way to undo what Thanos has done. By going back in time to retrieve the stones at various times before Thanos found them, they will be able to reconstitute their own glove and undo everything. So they all embark on this mission and succeed thanks to the sacrifice of several Avengers including Tony Stark and Natasha Romanoff. At the end of the movie, they have to take the stones back to where they took them so as not to change the timeline. Steve has to bring one back at the end of the 40s. But when he comes back, he is a very old man. He has chosen to stay in the 1940s with his lifelong love Peggy Carter. He has lived a long life with her and has been waiting for this very moment to come and give Sam his shield to become the next Captain America.