One thing that Marvel movies have going for them is their incredible commitment to continuity. A small, seemingly insignificant thing will happen in one MCU movie only to have enormous repercussions for the wider Marvel Cinematic Universe later on.
For example, in Age of Ultron, when Captain America grabs Thor’s hammer, and it moves ever so slightly, which comes back around four years later when Cap uses Mjolnir to ruin Marvel villain Thanos’s grand plans.
As the MCU’s grown, though, some plot threads from the superhero movies and Marvel series have been dropped. This is sometimes because the grand Marvel plan has changed, but occasionally, you get the feeling that the studio doesn’t want to deal with it. And there’s no greater example of this than when Hulk revealed Ant-Man is immortal.
Wait, is Ant-Man immortal?
Yeah, in a weird way, Ant-Man is kind of immortal, or at least he could be if he wanted to be (and we don’t mean in a Paul Rudd doesn’t age kind of way). You see in Avengers: Endgame, while experimenting with the cosmic tunnel, Hulk sends Ant-Man on a trip through time. This has the unfortunate side effect of turning Scott into a prepubescent kid.
Slightly worried, Hulk sends Scott back through the tunnel, and when he returns, he’s now an old man complaining about his back. With the Hulk now panicking, he again engages the time machine and accidentally turns Ant-Man into a baby.
At this point, Captain America steps in, and Hulk reverses the process by killing the power to the machine at just the right time, restoring Scott to his correct age (more or less).
The Avengers invented the fountain of youth
This never comes up again in Endgame, mainly because our heroes are a little preoccupied with their time heist, but it’s worth pointing out that the Avengers invented the fountain of youth.
In theory, using Pym Particles, the team could de-age themselves when they started getting too old or illness set in, essentially resetting themselves to their prime. They wouldn’t be wholly immortal and could die in battle, but they could live an ageless life if they wanted to.
This opens a whole can of worms in terms of storytelling. None of the Avengers should ever die now because there’s a semi-reliable way to restore them when they get hurt or too old. Think of the aged Captain America we meet at the end of Endgame. There’s no reason why they couldn’t have restored Steve Rogers to his prime.
So why don’t the Avengers use the machine?
The thing is, it’s not really that simple. From an in-universe perspective, it’s clear that time travel is far too dangerous a tool to be used to extend someone’s life, and as for Cap, well, he’d lived a good life. He didn’t feel the need to go back to war.
It’s possible that Hank Pym, who was fiercely protective of his tech, also refused to let the Avengers use his particles again. Out of universe, however, there’s a more straightforward explanation. It was a joke in an action movie, and we’re overthinking it. So no Ant-Man isn’t immortal, even if he could be.