How did Bucky not age?

Bucky Barnes did not age due to the combination of Hydra's experiments and the super soldier serum that was used on him, along with his time in cryogenic stasis. Here's a breakdown of the key factors that contributed to Bucky's lack of aging:

1. The Super Soldier Serum

• Like Steve Rogers (Captain America), Bucky Barnes was subjected to the Super Soldier Serum as part of Hydra’s experiments during World War II. The serum is designed to enhance a person's physical abilities, including strength, agility, and healing, while also dramatically slowing down the aging process.
• The serum gives the user a form of near-perfect health, effectively halting or dramatically slowing the biological aging process. While Steve Rogers' serum made him appear to age only very slowly, the effects on Bucky were similar—his physical age essentially remained frozen after he was injected with the serum.

2. The Winter Soldier Program and Cryogenic Sleep

• After Bucky fell off the train during a mission in Captain America: The First Avenger, he was recovered by Hydra and subjected to further experimentation. They turned him into the Winter Soldier, a brainwashed assassin with cybernetic enhancements.
• Bucky was placed in cryogenic stasis (frozen in a kind of suspended animation) between missions for many years, which kept him in a state of physical preservation. This not only prevented him from aging but also allowed Hydra to "reprogram" him and use him as a weapon over many decades.

3. Combined Effect of the Serum and Cryostasis

• The combination of the Super Soldier Serum and the cryogenic stasis explains why Bucky did not age over the years. The serum slowed his aging, and the stasis kept him physically preserved whenever he wasn’t active as the Winter Soldier. Even though Bucky spent decades in this condition, he appeared to be the same age in Captain America: The Winter Soldier (2014) as he did during World War II.

4. The Impact of Time on Bucky

• When Bucky is eventually "reawakened" in The Winter Soldier, he appears as a man in his 20s or 30s despite having been born in the early 1920s. His appearance hasn't changed much because of the combined effects of the serum and stasis, even though the events are happening decades after his presumed death.
• By the time of Avengers: Endgame, which takes place around 70 years after Bucky's fall from the train, Bucky is still physically in the same condition, looking nearly the same age as he did in the 1940s.

Conclusion:

Bucky's lack of aging is a direct result of the Super Soldier Serum he received, which slowed his aging process, and the cryogenic stasis he was placed in by Hydra between missions. Together, these factors kept him in a preserved state, effectively allowing him to survive and look the same for decades, despite the passage of time.