By the end of Avengers: Endgame, Captain America has retired from the superhero life, becoming old by spending a lifetime with Peggy Carter - but a time travel gag in the same movie, relating to Hulk's experiments, suggest that age may not disqualify the First Avenger. When first debuting an effort to turn the quantum bridge into a time machine, Hulk forces time through Ant-Man, instead of sending Ant-Man through time. The result, while comedic, suggests that this same application could be used for any person, and as Captain America has successfully used the completed version of the time machine just as well as Ant-Man did, there is no reason to believe that the same principle could not apply.
With Chris Evans having signed on for new projects within the MCU, questions have been raised about the capacity in which Captain America might return. The idea that an alternate Captain America from the multiverse might be substituted is also a strong contender, but the original Captain America is still around, simply old enough that he wishes to retire. With this potential remedy, it is possible that Captain America could shift into semi-retirement, still allowing his original mantle to pass on to a new candidate.
All of the technical clues that this time-manipulation trick could really work are found within Endgame. During Hulk's experiment, Ant-Man is disoriented by the changes that are overtaking him, but seems to have a continuity of understanding - he is not confused about where he is, or who the other Avengers are, even when regressed to childhood. Similarly, there is every indication that the adjustments are permanent until reversed; when Ant-Man is regressed to infancy, Hulk says, "He'll grow!". Peggy Carter is unlikely to return with Cap - she died several years back in continuity, and there's no evidence that the time machine can reverse that as well - but Captain America could have some youth back whenever he wants it.
It's for that reason that Steve Rogers is unlikely to reclaim the title of Captain America, regardless of his age. Having passed along the mantle of Captain America, he could take up the persona of the Nomad, allowing the new Captain America to stand in the spotlight. The series would still move on, and the transition of the title would still be meaningful. Since the upcoming project The Falcon and The Winter Soldier focuses on Cap's legacy, it makes the most narrative sense to have Steve Rogers as an occasional pinch-hitter. The emotional beat of seeing the familiar hero on the screen would still be considerable.
Ultimately, there's no certainty which version of Captain America will return. Any of the options - from Old Steve, to a multiverse double, to even a clone or robotic replica - are possible uses for Chris Evans' planned return, and with the plethora of available storylines to draw from and narrative tools at their disposal, it could well be that the character will show up in some as yet unimagined capacity. If they wish to use the original Captain America however, all the tools that they need are available in a one-off gag from Avengers: Endgame.