After being beaten by Thanos in Infinity War, the Hulk will get his chance at a rematch come Avengers: Endgame. But what if Hulk's defeat isn't due to Thanos being stronger, or more experienced... but because Marvel quietly aged Hulk thousands of years without fans noticing?
Whatever the reason for his defeat, nobody was more surprised then Hulk himself. After making a heroic return in Thor: Ragnarok, Hulk's loss has launched Bruce Banner on a brand new path, now that his green alter ego isn't coming out to play. Since there are hints that Hulk IS part of the Endgame mission, that's going to change. But if he's really as ancient and immortal as the MCU has apparently made him, it may actually be wisdom and experience that give Hulk an advantage over Thanos. Hulk's body may be failing after millennia spent fighting, but Bruce Banner's mind is still smart as a whip.
As hard as it may be to believe, we're willing to bet that fans will fully accept Hulk being immortal, let alone thousands of years older than the Avengers if it means merging into Professor Hulk (uniting the best of both Banner and Hulk). And if the heroes manage to defeat Thanos, then Hulk will get over his failing strength in time. But if fans are wondering where our assumption comes from of Hulk's true biological age, the mystery isn't difficult to solve. Fans just weren't looking for it.
The absence of Hulk from the MCU is thankfully now a distant memory, even if the movies haven't actually dealt with the reason he abandoned his team at the end of The Avengers: Age of Ultron. While Bruce Banner would have had reasons to run away from his guilt over creating Ultron, it was actually Hulk who set a Quinjet on cruise, and sat back to enjoy the stars. And it was Hulk who was still in control when Thor discovered him on the planet Sakaar in Ragnarok. It was eventually explained that Hulk's Quinjet had indeed entered space, gotten sucked into a wormhole, and spit out on Sakaar.
While Thor was seen as a piece of trash, a being like Hulk caught the eye of The Grandmaster, and soon became his champion of the gladiator arena. There he fought until Thor showed up, woke Banner up to reclaim control, and realized the good doctor had no memory of the two years that had passed since Age of Ultron's final scene. Well... two years on Earth.
But Thor: Ragnarok makes a point of showing how time passes "differently" on Sakaar when Thor and Loki wind up stranded on the planet after falling through wormholes of their own. That's thanks to Hela managing to tail them through the spatial bridge formed by Bifrost--tossing each of them free from its beam and into wild space. For Thor, his travel to Sakaar is instantaneous. But when he meets Loki, it's revealed that while the God of Mischief left Bifrost just moments earlier, he arrived on Sakaar "weeks" before Thor. When you do the math, that difference means BIG things for Hulk's own time spent on the cosmic junkyard planet.
Doing the exact math, Loki departs the Bifrost to be gathered up by a wormhole and spat out onto Sakaar exactly 11 seconds before Thor, resulting in "weeks" of difference. If we assume a minimum definition of "weeks" as just 15 days, that means one second of time outside of Sakaar (be it on Earth, in space, or any other parallel plane) equates to 117,818 seconds on Sakaar--or 1.36 days. That might not seem like an outrageous case of relativity, but taking into account the full two years Hulk was missing, that means 235,000 years passed on Sakaar while the Avengers were living through just two. If Loki was referring to a stay closer to say, 22 days, one second would equal two days in Sakaar... that's 345,000 years.
The movie makes a point of confirming the warping of time on Sakaar when The Grandmaster states that "Time works a little different around these parts. On any other world, I'd be like, millions of years old." That age certainly fits with our own calculations, and wouldn't actually have to refer to biological time at all. In the modern Marvel Comics, The Grandmaster and the Hulk are BOTH immortal. Bruce Banner wouldn't know the difference locked away inside Hulk's mind, but Hulk's immortal hair turning to grey proves even his power has limits.
There is obviously wiggle room in the physics at work here: how does time pass differently through different wormholes, would Hulk's mass alter that relativity, and when exactly did he leave Earth's orbit? But no matter what, it's a question of tens of thousands, if not hundreds of thousands of years Hulk spent on Sakaar. Will all that experience be used for what comes next in Avengers: Endgame? Did Hulk actually learn anything useful along the way? And will the Avengers ever actually learn that Hulk is now a more ancient warrior than any they've fought thus far? Only time (and Avengers: Endgame) will tell.