We don't know the exact details, but at the end of Avengers: Endgame we see
Captain America going back to return all the Infinity Stones to their time lines. After that, he chooses a specific point in time in which he can have a "normal" life with Peggy, thus, creating another branch in the flow of time. We know from other questions/answers that he found some other way to go back to the main time line, and have that conversation with Falcon.
However, when he
returns to the main time line, he is old!
Why did that happen? Does he follow a regular process of aging as any other human being? Or did he find a way to extract the super serum injected to him in the 1940s?