In World War Hulk, we see a what-if scenario with Bruce Banner actually making it to the 'peaceful planet'.
Here we see him become the guardian of a lizard-like species that eventually evolves into sentience, a process that should take bajillions of years.
Does this imply the Hulk is immortal? What about Bruce?
The Incredible Hulk(s) - (She-Hulk, Red Hulk): while it has never been substantiated, the Hulk's near-perfect and instantaneous regenerative abilities should mean he will never appreciably age or will age incredibly slowly in relationship to other humans.
It is safe to assume all Hulks (red or green) will have the same nearly-immortal quality. It is unknown whether any of the Hulks will/do age while in their human forms.
The Hulk Recovers from Vector's devastating attack - Incredible Hulk vol. 2 #398
The mechanism for the Hulk's biological process has not been explained or developed so we are forced to determine how his biological process functions from examples shown in the comics. As the above example shows, the Hulk is able to regenerate from injuries that would kill ordinary humans. He is able to regenerate mass and repair tissue damage nearly instantaneously. This would give the impression he would have the ability to have a very long, if not immortal, lifespan.
As far as Banner's aging is concerned several theories have been forwarded over the decades in the Hulk's books, occassional What-If's, one shots, and alternative timelines. They include:
As Banner aged Hulk would simply stop turning into Banner after he grew too old to sustain himself effectively.
Banner is as immortal as the Hulk deriving his immortality from their shared cellular relationship
Banner dying physically, perhaps from a heart attack, and turns into the Hulk unable to return to his potentially human form.
What if scenarios do not exactly replicate the canon characters.
The goal of these stories function as a Gedankenexperiment (though-experiment) a method to extrapolate characters, ideas, themes outside of the canon universe.
If a representation of a character shows up with a power or ability not necessarily seen in the canon character it may be implied the canon character could have this ability but there is no guarantee.
Sometimes a comic company will adapt an idea or power from an experimental story and bring it to the canon universe, but it is done on a case by case basis.
In another one-shot "Hulk: The End" written by Peter David, immortality is also implied for both of them. The Hulk knows that if Banner dies, he dies as well. So whenever Banner is near death, the Hulk takes over and heals their body with his regenerative powers. Essentially keeping Banner as immortal as he is.
I don't believe it's canon but it might shed light on how Banner might become immortal due to the Hulk's powers.
Logically it would hold that the Hulk is immortal, since the comment on extra-cellular damage being more a cause of aging than the cells themselves wasting away does nothing to deal with the fact that a bullet in one's brain functions more or less the same way (in the sense of being damage caused from outside the actual cellular and mechanical framework of the body). As a sidenote, to the idea of the Hulk not having a flaw, as in greek tragedy, hubris is not necessarily a condition of arrogance, it can also often follow directly from the conditions of the tragic character's strengths, ala Oedipus' mind, or the Hulk's infinite, and therefore uncontrollable power.
Banner is mortal, but can transform after any injury that doesn't cause INSTANT death. The Hulk has a healing factor greater than Wolverine's and so, (as Logan does not age), we can assume that Hulk does not age, (though Maestro does suffer a bit of pattern baldness in extreme old age). But when Banner transforms into the Hulk, any cellular damage due to aging is repaired, therefore the Hulk transforms back to a human in their prime and in perfect health. This is made clear in the Ang Lee movie (I know ... groan) "Remember my bad knee? ... Well, now it's my good knee." The Hulk WILL heal and it is illogical to think he will transform back into a 'damaged' human. As long as Banner keeps turning into Hulk, he will continue to regenerate into a young man and therefore, remain an immortal (the kind that can be killed, if you are quick).
Yes but not absolutely as he could still be killed by reality warping beings like Thanos with the infinity gauntlet or Pre-Retcon Beyonder. Thanos could use the time gem to revert the Hulk back into Bruce Banner. Once the Hulk had been turned back into Bruce Banner Thanos could use the Power Gem to deliver a devastating blow thus killing Banner instantly. The Beyonder a being so powerful that he can literally erase the multiverse and by erase I mean erase! Too destroy the Hulk he would literally just need to think about the Hulk dying and it would happen. The hulks immortality well at least very long lifespan comes from his regenerative healing factor that allows him to heal from any kind of damage. Too give some purely hypothetical scenarios to show the extent of the Hulks healing abilities (not to sound like some new age cultist). Banner could have fallen off a cliff to his doom but instead of dying when he hit the bottom he would transform into the Hulk thus saving his life. Another more extreme example is that Banner could be ripped apart instead of dying like most people the Hulk would immediately take over and "heal" Banner causing him to come back to life. Even if Banner was thrown into the sun the Hulk could easily leap out and smash who ever threw him in. The Hulk has zero limits to his healing ability as all he has to do is get angry enough and he'll instantly be able to heal from any kind of damage. So there you go hopefully that answered your question at least partially.
