The MCU (Marvel Cinematic Universe) has become an unstoppable life force. The studio boasts an impressive archive of 23 movies to date, with at least 14 more in various stages of development. As the movies gained a global following, the characters reached cult status among fans.
Steve Rogers (aka Captain America) is one of these characters. After his happy-ending departure in Avengers: Endgame, an assortment of fan theories about Steve’s life after the shield made the rounds on various forums and fan boards.
That said, an interesting time-travel theory appeared on Reddit a few weeks ago.
The theory recalls Captain America’s decision to go back to the past and stay with Peggy Carter.
“When Captain America returns to Peggy Carter at the end of Endgame, he’s around 35 years old. The year he went to is presumably 1945, (The year when The First Avenger takes place). Meaning when the events of the first Iron Man movie happen (in 2008), he is around 98 years old. Stan Lee was 85 when that movie was released. If you account for the fact that Captain America has superpowers, he probably looks much younger than most people do for his age, so it’s not too far of a stretch to say that he could look 10 years younger. My theory is that Captain America, who was getting old, decided to check up on his friends every once in a while. Whenever he did so, he put on a disguise of sunglasses and a moustache, so as to not cause any time paradoxes or complications. This explains why the same character keeps on popping up in close proximity to the Avengers, as it is just Captain America seeing how his friends are doing.”
The Captain America theory makes reference to Stan Lee’s cameos that fans were treated to in every MCU movie, before his passing. There are numerous scenes where Lee appears (mostly) in close proximity to the main heroes.
While the concept is pretty fun, the theory doesn’t really hold up to much scrutiny.

As far as we know (and as dictated by the rules of time-travel), Steve returned the Infinity Stones to their proper places and went straight back to the ’40s to be with Peggy. He didn’t do any sightseeing in between.
As for Lee, Guardians of the Galaxy Vol. 2 already hinted that he was working for the Watchers in some capacity. Maybe as agent meant to keep an eye on Earth’s Mightiest Heroes?
Most importantly, his cameo in Captain Marvel also suggests that he actually is Stan Lee. Excelsior!!!