I am wondering this, as it is a fascinating question in it's own right. I know that Heracles lived at about age 70-80, but what is the minimum, if not the maximum age a demigod can live.
In the absence of any statements to the contrary, I think it is safe to assume that demigods have normal human lifespans for people of their rank and profession (i.e. Bronze Age soldiers and royalty). I can't recall any myths that mention, for example, a demigod dramatically outliving their non-demigod spouses, the way that Arwen is foretold to outlive Aragorn in LOTR.
Worth mentioning: similar to modern superheroes, ancient Greek Heroes were known to get into some shenanigans (e.g. entering and returning from the Underworld, ascending Olympus to become a true god, etc) that can make it tricky to even calculate their age at all. (Is Captain America 100 years old, or does being frozen for decades mean he's still just in his 30s? Did Herakles die in his 70s, or does becoming a god not count as dying?)
Ancient Greek myths also have fluid timelines, again similar to comic books, so you can't precisely date a Hero's lifespan based on external events, just like you can't pick a specific date for Batman's birth because his stories keep getting updated over time.
But overall, it seems that demigods age and die at about the same speed as normal mortals, unless they are killed in battle or murdered.
Presumably by "demigods" you are referring to the relatively modern term as taken to denote a character from Greco-Roman mythology who is the offspring of a deity and of a human being.
The Ancient Greek hēmítheos and the Latin semideus, from which we get the English "demigod," does literally mean "half-god," but this is misleading. The ancient usage of the term does not refer to a mortal who is literally half divine based on his/her parentage. Rather it is a term for a lower-tier divinity, a sort of minor god.
In Ancient Greek religion one can sketch out roughly three different levels of divinities who were honoured with cults: the theoí (gods); the hēmítheoí; and the Hḗrōes (Heroes). For the most part, the theoí were born immortal and had always been "the gods."
Hēmítheoí and Heroes were two different classes of divinities who typically tended to be dead humans who were recognised for their great deeds in life and therefore considered, in the afterlife, to have become powerful entities although lesser than the theoí. And it did so happen that a majority of them happened to have at least one divine parent.
Moreover the differences among these three different levels was not a permanent configuration, as demonstrated by the myth in which Herakles [Heracles] starts out as a Hero and then works his way up gradually to become a hēmítheos and eventually a full-blown theós, or, perhaps, more strictly speaking, he is considered to inhabit all three categories at the same time.
In one story, after Herakles’ suicide, his remains could not be found on his funeral pyre, so his nephew Iolaos became the first to sacrifice to the dead hero as a hemitheos (demigod).
According to Pausanias’ Description of Greece, Herakles had a son named Phaistos [Phaestus] who became king of the city of Sikyon. When Phaistos first arrived at Sikyon he found that the locals were making offerings to Herakles as a hero.
Phaistos then refused to do anything of the kind, but insisted on sacrificing to him as to a theos. Even at the present day [
100s AD, over a thousand years after Herakles’ death] the Sikyonians, after slaying a lamb and burning the thighs upon the altar, eat some of the meat as part of a victim given to a theos, while the rest they offer as to a hero.
As far as I am aware, the only extant ancient source that mentions anything about Herakles' age at death is Book 2 of Clement of Alexandria's Exhortation to the Greeks, which says that Herakles had lived for fifty-two years when his body burned on Mt Oita [Oeta]. (The ambiguous wording in the passage might possibly be implying that Clement's source for this detail is either one of the Peripatetic philosophers Hieronymus or Dicaearchus.)
Even going by the more modern Riordanesque definition of "demigod," there is nothing particularly special about the lifespans of the children born from unions between humans and the Greco-Roman gods. For the most part, generally speaking, in terms of capabilities, durability and nature, they are just as mortal, vulnerable and human as all other ordinary men and women, pretty much indistinguishable from them.
Among these mortal progeny of the gods there are indeed some who stand out in a few different ways, but they are exceptions. Especially as far as lifespan is concerned, this is best illustrated by another son of Zeus, namely King Sarpedon of Lycia, who was slain in the Trojan War, despite Zeus's own desire to keep him alive. According to Apollodorus' Bibliotheca 3.1.2, Zeus had created a special dispensation for him to live for three generations, thus making him most likely the oldest person to fight in that war.
Glitches in the Timeline
Even ancient mythographers seemed to be a little bit incredulous at the idea that Sarpedon, who should have died something like a hundred years before the Trojan War, was one of the warriors defending Troy. This gave rise to two different versions of the character. In Homer's Iliad he does not seem to be a supernaturally old man, being the son of Zeus by Bellerophontes' daughter Laodameia. There is, however, also the version in which he is the son of Zeus and Europa, and thus one of the brothers of King Minos of Crete. In yet one other variant the Sarpedon who dies at Troy is the grandson of Minos' brother Sarpedon.
The mythology is somewhat replete with chronological discrepancies like this one. Minos himself is supposed to have died only one generation before the Trojan War. However, if he was the son of Europa, then by my calculations we would have to have been at least two hundred years old at death. Again, certain ancient writers, being aware of the issue, came up with similar explanations as for those of his brother Sarpedon. The Minos who died a generation before the war at Troy, according to them, is the grandson of a more ancient Minos, who in turn was the son of Zeus and Europa.
On occasion, Herakles/Hercules also encounters individuals from long before his time in places like Egypt and India. The Roman writer Cicero explains this away by asserting that there were a number of different previous characters called Hercules, and some of them were children of Zeus (or rather of his Roman counterpart Jupiter) by yet other women.
It would be a clever hack to these anachronisms to deduce that these "demigods," because of their descent from the gods, are just that long-lived, and therefore could last a couple of centuries before growing old and perishing, but no ancient source makes such a claim. Hence all mortals, whether their parents are immortal or divine or not, are portrayed as suffering the same rate of decay as everybody else, unless extracurricular intervention is made on their behalf.
Minimum Life Expectancy
Whatever age at which a regular garden-variety human can die is the same age at which a "demigod" can end his life, which is why Herakles' stepmother Hera tried to assassinate him when he was still a baby. Although that attempt failed, there are, in fact, quite gruesome stories featuring the deaths of infant children of certain deities.
In one, an Argive princess named Psamathe was raped by the god Apollon [Apollo] and bore him a son named Linos [Linus]. Being unmarried and still living in her father's house, Psamathe had managed to keep the birth a secret, but fearful of what her father might do, she abandoned the newborn outside where he was torn apart by the king's sheepdogs. (This tragedy is part of a story told to explain how a temple of Apollon came to be built on Mt Gerania.)