Se você criar novas linhas de tempo quando mudar o passado, o capitão deve ter retornado a esta após envelhecer.
Pelas regras do universo,
you cannot alter current timeline (that is why they couldn't kill Thanos as a baby).
Então, quando o Capitão ficou com Peggy, ele criou e viveu em uma linha do tempo alternativa. Presumivelmente, tudo aconteceu de maneira semelhante a esta.
Isso é suportado por este entrevista.
Cap did, in fact, create an alternate reality, one where he lived his life with Peggy.
Q: Peggy Carter was probably already married and in her mid-40s in 1970, in that case, what year was it that Captain America went back to dance with her?
A: We can’t answer it, for now, this is a story that happened in an alternate reality. [...] The time travel in this movie created an alternate reality. He lived a completely different life in that world. [...] 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.
E em algum momento, ele teve Pym, Scott, Stark e Banner, da sua linha do tempo, para trazê-lo de volta a este. Ou talvez ele tenha usado outro método desconhecido para nós, espectadores.