The Hulk's regenerative powers seem without limits. Does this imply Hulk doesn't age, can't die and can't be killed permanently by any conventional means?
Being the Hulk means never having a last birthday party.
(Not that he's into parties anyway...)
1. Hulk's regeneration factor is not in fact the strongest in the Marvel Universe; the Sentry's, for example, is stronger.
2. From page 117 of Hulk: The Incredible Guide (written by former Marvel Editor-in-Chief Tom DeFalco, so you know it's good; I'm just going with it because it's a convenient source for me to cite): "The Hulk could survive almost any catastrophe. Banner, however, was only human. Though he has somehow stayed alive for more than 200 years [elsewhere on the page it notes that his actual age is unknown, so this should be considered a lowball], his body was finally running down. He awoke one night, feeling as if his chest was on fire, and he knew that his suffering was almost over. Fighting like a caged animal, the Green Goliath broke free, refusing to surrender to death. The Hulk realized at once that something was different. Bruce Banner was dead. Knowing that he, too, would die if he ever changed back into human form, the Hulk sat and waited. He had always wanted to be alone... and he had finally gotten his wish. " This means that while Hulk's regeneration applies to Banner (allowing him to recover from being shot in the heart, for example, as noted on page 110), the transfer is not perfect and does not completely prevent aging, that Hulk and Banner can die independently of one another, and that Hulk is eternal, if not ageless, as near as can be told.
3. From page 95: "The Maestro failed to corrupt the Hulk. Knowing he couldn't beat his future self in physical combat, the Hulk used the time machine to send the Maestro into the past. The same gamma bomb that originally created the Hulk incinerated the Maestro. " So Hulk can in fact be killed, but no word on how specific the required conditions are.
4. From page 93 "[Speedfreak's] arm sheaths contain adamantium swords that are sharp enough to decapitate the Hulk. " No word on whether this would prove lethal, however.
In the Next Avengers animated movie, after Ultron pretty much dominated the world and defeated to the death most all our beloved old School Avengers (Ant-Man, Wasp, Black Panther, Hawkeye, Cap, Black Widow, Thor) the tween Avengers embark on their journey to Avenge their parents, and meet up with Old Man Banner and his alter-ego, White-Haired-and-Balding Hulk, who takes Tony Stark's Iron Avenger bots, previously co-opted by Ultron (smooth move, Iron Man: design and build robots to fight an effectively omniscient, planet-engulfing super'bot, that'll surely work out well, smart choice!) all to Tuna Town. This would imply that Hulk does, in fact, age, and does it gracefully! It appears that old age doesn't slow his roll as far as the Rage=Strength equation goes, though...not to mention, the Maestro is a version of the Hulk who survives long enough to learn and devolope a personality, albeit an Eeeeeeevil one. Can he die of old age? Not likely, since the Hulk sustains hinself through gamma rays as much as we do through food, if even that. As far as conventional means of slaying a being....No. Just No. Like, bullets and bombs? So much no. It would require application of such by a force at least as strong as the Hulk, and if thats the kind of power level we'd need, then "conventional weapons" are so far below the mark they would be inapplicable to this discussion.
There was a story on this called Hulk The End. The Basic plot is that a nuclear war has wiped out nearly all life on Earth which includes the heroes and villains. The only surviving life forms are mutated insect creatures which come to feed on the Hulk occasionally. Banner reflects on the war and feels very lonely, he wants to die now. Later on he suffers a heart attack but the Hulk takes over. Hulk decides he will never revert to Banner again as he will die and Hulk wants to prove that he is the strongest on there is.
Another example is from Old Man Logan (Great book by the way) where the villains have won against their heroic counterparts and Wolverine has given up being a hero and decided to just fit in this world. Certain events lead to him confronting an old Bruce Banner who now possess super strength in his human form as well. There is no real change to the Hulk apart from the fact that he seems to have gotten stronger than he was before.
Also there was Maestro from the future imperfect storyline who was Hulk from an alternate future. This Hulk is evil and appears to be stronger than the traditional Savage Hulk we see.
From most of these non-canon stories we can assume that there wouldn't be any real change to the Hulk apart from greying hair and wrinkles. If is about to die we can assume that the Hulk will take over like he did at the end of Hulk The End. Most future versions of the Hulk seem stronger but that was due to absorbing radiation from nuclear wars which occurred in their reality.
In recent comics book civil war II, Hulk's alter ego Bruce Banner is killed by Hawkeye
It is pretty unlikely.
The only way that might work for sure is for Bruce Banner to die exceptionally painlessly and exceptionally quickly. If Bruce was fully anesthetized into a medically induced coma while he was already asleep and did not know what was happening and then you killed him near instantly with anything other than radiation he *MIGHT* die.
That is about the best shot you got and even that would likely fail somehow. Even then, deaths in comic books are almost never permanent especially for long standing characters like the Hulk. He might die but someone would travel all the way to the underworld if need be (becuase only the Hulk can kill <X>) or he'd be kicked out of the afterlife for destroying the peace of heaven with his anger.