Steve Rogers was more powerful than he looked during his final sequence featured in Captain America: The First Avenger, also looks to be exiting the Marvel Cinematic Universe.
In the aftermath of Thanos's defeat, Steve volunteered to return the Infinity Stones to their rightful locations among the timeline. Rather than return to the date in which he left, Steve decided to travel back further into the past. He took advice from the late Tony Stark and lived a full and happy life with his former love, Peggy Carter. When he appeared in the present timeline, Steve was no longer the embodiment of the ing over the Captain America mantle to Sam.
Based on the murky elderly Steve was around 110 years old, but the character was technically alive for closer to 180 years. Steve might have given up his shield, but that doesn't take away the fact that he was still much stronger than an average human. Even well into his 100s, the former Captain America only had the body of a man in his 70s or 80s. His delayed aging shows that the Super Soldier Serum still had its advantage in of power.
Old Steve Rogers Still Had Super Soldier Powers
After acquiring the Super Soldier Serum in The First Avenger, Steve obtained powers such as enhanced strength, durability, stamina, and senses in addition to regenerative healing. With the regeneration, Steve was immune when it came to infections and disease, which in turn, affected his longevity. The character didn't age at a normal rate, explaining why he didn't look to be on the verge of death in Endgame despite his advanced age.
Even though Steve still had the serum coursing through his veins, the powers were working at a declining rate. He still carried his superpowers, but they wouldn't have been at their peak. That said, his strength and potential in his elderly state would have been much stronger than an average human, and possibly greater than some of his fellow Avengers. Steve certainly would have continued being worthy enough to wield Thor's hammer in his elder state, which could have been an exciting sight to see.
Despite Steve's continued strength, he felt an emotional need to step away from the Captain America mantle. Technically, Steve could still serve as a key member of the MCU franchise, playing a mentor to the next generation of heroes. With decades left to be lived, it's unclear what lies ahead for Steve after Avengers: Endgame. Either way, he's not a typical old man, and those Super Soldier Serum powers are still there if need be.
