Marvel Comics already has the perfect way to bring the young version of Captain America back to the Marvel Cinematic Universe, should they choose to continue on with the character. In the comics, Steve Rogers was once turned into an older man by the villain Iron Nail but later returned to his normal age after encountering a sentient being created from the Cosmic Cube. The comic lays the groundwork for how Captain America could come back in future MCU projects.
Captain America's final chapter in the MCU featured the hero choosing to go back in time and return the Infinity Stones and Mjolnir to their rightful places throughout history. By time-traveling to the past, Steve Rogers was allowed to experience everything he missed after being frozen in ice - and got his chance to dance with Peggy Carter. In the present timeline, Steve became an elderly man, and ultimately passed on his shield to Sam Wilson, naming the hero the successor to Captain America. It was a pitch-perfect ending for Rogers, but if Marvel has more plans for the character and needs to de-age him, two comics show how that would be possible.
In Captain America #21 by Rick Remender, Nic Klein, Dean White, VC's Joe Caramagna, Steve Rogers faces off against the Iron Nail, a former S.H.I.E.L.D. Agent trying his own attempt to overthrow powers in the West. Captain America managed to save countless lives and stop the villain. Still, in the process, the Iron Nail managed to neutralize his Super-Soldier Serum, which caused Steve Rogers to age into an older man rapidly. While the events would lead to Captain America passing the mantle onto Sam Wilson, eventually, the hero returned with the help of the Cosmic Cube.
In Avengers: Standoff!, old man Steve Rogers is nearly killed during a fight against Crossbones. However, as he's about to get hit by his own shield, Kobik - a shard of the Cosmic Cube who has gained sentience and taken the body of a young girl - offers to make him a hero again. She uses her powers to de-age him and bring back his superpowers. Considering Captain America went back in time to return the Infinity Stones, Kobik could exist as a fragment of the Space Stone. Perhaps, his close handling of the Stones could help create her or changed him from within.
It's doubtful that the MCU brings Steve Rogers back as Captain America. Not only would it take away from the current Captain America, Sam Wilson, and his time as the hero, but it would also rewrite a perfect ending for the character. However, if the Marvel Cinematic Universe ultimately wants to bring back Captain America, they already have the source material to make it happen.