In Avengers: Endgame, Hulk invented a machine that can make people immortal, although it was by accident. I mean, while testing the time machine, he de-aged Ant-Man and if they didn't reverse that process then he could have lived his life from the beginning again. And now if Steve Rogers wants to dance with Peggy then also that promise was fulfilled, so why didn't Steve want to become Captain America again?
If Steve Rogers wants to dance with Peggy then also that promise was fulfilled, so why didn't Steve want to become Captain America again?
Steve didn't want to become Captain America again. He's served his time AND managed to have a full life with Peggy as well. It's time for someone else to take over.
Consequently, there was no need to make him young again (even if that were actually possible to do deliberately, which is doubtful) because that's not what he wants.
Of course, Out-of-Universe, Chris Evans has been reluctant to continue in the role (and his contract was up as I recall) so swapping out the actor but continuing the character makes this obvious.