Across A Billion Years by Robert Silverberg features archaeologists searching for artifacts of an ancient race which disappeared about that long ago. Throughout the novel, they first discover some sentient robots, and, toward the end, the remnants of the race itself... a few thousand mindless husks on life support. It has been fifty thousand years since one died. Four million years since one was born.
Age of Fire: The dragon NooMoahk is one of the oldest living creatures still in existence by the time the series starts, and can remember times when humans had not yet mastered metal, the now-savage blighters ruled much of the world, and once-extensive glaciers crept north to be replaced by forests and then dry wastes in turn.
Dwellers from The Algebraist by Iain Banks are old enough, and patient enough, to have populated the entire galaxy. Without faster than light drives. Individuals are explicitly said to be billions of years old, which means that dwellers are essentially a first-generation society.
The Chee are androids who came to Earth before human civilization and have since lived through most of human history. Erek, the one we see most often, casually mentions that he helped build the pyramids, among other things. (Though he just helped haul stones, an interesting subversion of the Ancient Astronauts theory.)
The Ellimist, who was already millions of years old when Earth was just beginning to form.
Anita Blake: Circus of the Damned, the third novel, features a vampire named Mr. Oliver who claimed to be "older than time" which she took as hollow boasting until she realized he wasn't a human vampire, but a Homo erectus vampire. Due to the Sorting Algorithm of Evil, he went down far more easily than many enemies since then, making him rather disappointing in retrospect.
The Watchers, mysterious beings who predate human civilization itself, and may be a sort of fallen angels related to the setting's demons. They are effectively immortal, have extremely powerful magic and are revealed to be running an Ancient Conspiracy that seems to be steering both Amorr and Savondir into conflict.
Some of the older elves approach this. King Mhael's grandfather is said to have ruled seven kingdoms when the races of men were still naked bands of hunters in the woods, which would be at least many thousands of years in the past. However, the Watchers were already ancient when the elves were primitive tribes.
R. Daneel Olivaw, AKA Eto Demerzel, AKA Chetter Hummin is 22,000 years old at the end of Foundation and Earth. Gaia, a living planet is also very old, but younger than Daneel. Its exact age is unknown, but is estimated to be roughly 19,000 years.
The short story The Last Answer (not to be confused with the vastly more famous The Last Question) features a timeless entity that says it has no beginning or end, and simply is. Notably, the entity itself is not entirely pleased with this; for this reason it has created the entire universe, from which it harvests minds that it forces to think — eternally — for a way of ending itself.
Speaking of The Last Question, the AC. It has the memories of the first big AC on Earth. The story ends some unspecified (but extremely long) time after the heat death of the universe.
Jane Lindskold wrote the Athanor duology, where the title character of the first book, Changer, was literally one of the first two life forms on the planet, even before there were continents.
Zarathan, the setting of The Balanced Sword, has gods, demons, Ancient Saurans (Precursors who never went anywhere), and a few very long-lived wizards, so it happens more often than you might think that a character casually mentions how this reminds them of something that happened to them four hundred thousand years ago.
Any and all demon-like entities in The Bartimaeus Trilogy. For example, Bartimaeus used to be best friends with Ptolemy, and at that point, he was about 3000 years old.
Bazil Broketail: Waakzaam dates back to the First Aeon of the world, when life was just beginning to evolve.
The term is used explicitly in the tenth book of The Black Company by Glen Cook, as Blade ponders the perspective of the demon Shivetya, who is nearly as old as the plain of glittering stone between worlds and who may be as old as (or older than) the gods themselves. If the gods can be said to have properly existed.
Saetan, Andulvar, Mephis, Prothvar, Hekatah, and Cassandra from the Black Jewels series, all remember a cataclysmic war fifty thousand years ago, Cassandra more so than rest, as she is from a short-lived race among the Blood, meaning her natural lifespan should have been one hundred fifty, at best, compared to the five thousand of the long lived races, never mind Geoffrey, Draca or Lorn, the first being from an ancient race long, long forgotten even fifty thousand years ago, the latter two being dragons who created the Blood in the first place.
In Boundary's Fall, High Wizard Aemon (around five thousand) and Emperor Alwellyn of the elves (over six). In fact, Alwellyn is only the second Emperor the elves have ever had.
Robin Goodfellow in Rob Thurman's Cal Leandros series casually mentions his encounters with famous historical and mythological figures, from Freud to Bacchus. He also mentions being around since before humans came out of the caves. Although he never comes out and says it, it is surmised that he is hundreds of thousands of years old.
The Lost Technology spacecraft Rama in Rendezvous with Rama. However, the sequels show that Rama is actually much newer than it seems, or at least undergoes maintenance fairly often.
Codex Alera has Alera, basically the Anthropomorphic Personification of the country, who remembers the last few ice ages. "I have seen thousands of millions of years, Octavian."
In Cosmicomics, Qfwfq recalls memories from before the Big Bang.
Cthulhu Mythos: Characters frequently find themselves in awe and dread of what primordial secrets they learn about, and usually find at the end that something from those times still lives, or at least exists actively even if they're not alive in the usual sense. H. P. Lovecraft himself believed the universe to be eternal, which gave him the possibility of throwing around numbers too big to even fit the modern scientific view of the age of the universe.
Azathoth, seeing as he is the creator of the universe (not that he noticed it...)
In At the Mountains of Madness, the city of the Elder Things had been continuously inhabited since the Earth was young, for billions of years. The same applies for whatever it is that lies beyond the titular mountains.
"The Call of Cthulhu"" Casually exaggerated, as which the narration claims that Cthulhu is rising for the first time in vigintillions of years. Depending on whether it's in American or British notation, that's either ten-to-the-63rd or ten-to-the-120th years — either way, a vastly greater span of time than how long the universe has been around.
In "The Haunter Of The Dark", the protagonist writes down the history of an ancient artifact after gazing into it. Said artifact was originally created on planet Yuggoth (AKA Pluto), and came into the possession of the aforementioned Elder Things on Earth billions of years ago. Later it was worshiped by a race of serpent-men in ancient Hyperborea, until it was lost for millions of years before being found by the first humans in Lemuria and millennia later again in ancient Egypt, where the eponymous Avatar of the god Nyarlathotep was given its familiar name, and was involved in the fall of an entire dynasty. A few thousand years later it was found in the ruins of a temple and brought to Providence, where an occult cult formed around it.
"The Hounds of Tindalos": The titular creatures are from an age "before space and time".
"The Shadow Out of Time" plays this to full effect by having the older-than-humanity scroll written in the narrator's own handwriting. While the scroll's writer time traveled, the scroll itself, and what remains of the Great Race's city (not very much, really) are still over 200 million years old, and are both expected to last at least as long into the future.
Yog-Sothoth is the Gate. Yog-Sothoth is the Key. Time and space are one in Yog-Sothoth, for he is the All-in-One and the One-in-All. He knows and sees all that is, all that was and all that will be. He knows the answer to every question but only very brave or very foolish mortals would dare to ask him, for the price of ultimate knowledge is great indeed.
All other Outer Gods and Great Old Ones count too; while Yog-Sothoth is unique in existing in the past, present and future at the same time, all Outer Gods are impossibly ancient (older than all of space and time, in fact). Great Old Ones are millions or billions of years old too, although they have spent a portion of it in hibernation when the stars aren't right for them.
The Flowing Queen from the Dark Reflections Trilogy not only happens to be one of the old gods, who walked the Earth long before ancient Egypt, but also is stated to be older than any form of life in a sea.
Daughter of the Sun: Aelia has been in existence for fourteen billion years. She's a Goddess, so it's not as surprising as it might be otherwise. It then turns out she's even older.
Days Of Solomon Gursky begins with a normal human discovering the secret of living on after death via nanites as a result of a tiny choice he makes. He ultimately lives until the end of the universe as an increasingly powerful post-human but regrets having lost his wife. At the end of the universe he and other galaxy-sized super-beings use the collapse of reality to create new simulations of the universe, letting him outlive his own universe. However, the new universe is one in which he makes the opposite choice and doesn't discover immortality.
Pretty much any book about astronomy that deals with the future of the Universe will feature a Time Abyss. Such an example is Deep Time, a book that follows the life of a proton from shortly after the Big Bang to the very far future when it disintegrates because of proton decay, going even much further away with a positron born in the latter event.
The Salaxalan ghost in Dirk Gently's Holistic Detective Agency. He has been around literally since the beginning of life on Earth, which means that he literally spent two billion years surrounded by mud and "slimy things with legs". The time has driven him... a little bonkers.
In Going Postal, Anghammarad is a nineteen-thousand-year-old golem who finds work as a post officer. He expects to wait for the next universe (golems believe time goes in circles), so he will be able to deliver that one message he couldn't. As he says about "big green things with teeth". Bigger. Greener. More teeth. Quite appropriately, he is given the position of 'Extremely Senior Postman'.
Making Money: Not one of the Golems of Um is less than twenty thousand years old (they don't speak the same common language, and that's at least twenty thousand years old), and some may be as much as sixty thousand years old.
In Hogfather, the first Bogeyman reminisces about the days before men had fire and metal, when the continents were different. Anybody who lives in his realm will never die, simply fade away.
"You don't die here. You just get old... listening to the laughter..."
And then, of course, there is Death, who was there when the first lifeform died, and whose job it will be, at the end of the universe, to metaphorically put all the chairs on the tables and turn out the lights. Then there's his boss, Azrael, the Death of Universes, who can think on the answer to a question for long enough that a star can be born, live, die, and collapse into a black hole, and whose body parts are most easily measured in terms of the speed of light. According to Eric, Death won't just be there to metaphorically "close up shop" but he'll be there when the next universe starts up.
High priest Dios in Pyramids. At first glance, he is merely middle-aged or elderly. It quickly becomes apparent that he has some kind of personal knowledge of the past six thousand years of his country's history. He’s immortal thanks to periodic abuse of Pyramid Power and the conviction that the kingdom would fall to pieces without him. At the end of the novel, it's shown that he is, more or less, a living Stable Time Loop. In one sense, that means he's as old as ETERNITY, but he only experiences it six thousand years at a time. "Only."
In The Last Continent, the Luggage spends so many millennia buried under the soils of XXXX that a layer of opal forms on its surface. While this could be a case of sleeping through the Time Abyss, it's awake and bored enough to be even more pissed than normal when someone finally digs it up. Similarly, in The Science of Discworld, the Luggage gets stuck in the earth on our world for a couple of geological periods.
The Great A'tuin and the four elephants who stand on his back. How many millennia have they watched the play of stars and felt the feet of mortals walking upon their backs?
Doctor Dolittle's Post Office: Dr. Dolittle encounters Mudface, the turtle who was a passenger on Noah's Ark. In a posthumously published volume, Dr. Dolittle and the Secret Lake, we eventually get to hear the Mudface's story of the Great Flood which is alluded to, but not presented, in the earlier volume.
Domina: When asked how old she is, Silk says she is older than every single living human combined, which totals up to two trillion years. She then laughs, claiming to be far older than that. It's implied that this number includes all her bodies added together, so by a straight timeline she's likely significantly younger.
The Tale of the Five: The dragons introduced in The Door into Shadow can live to be thousands of years old and can remember clearly enough to calculate their ages by counting sunrises backwards. Furthermore, they also inherit the memories of their ancestors, although those memories tend to fade over a large number of generations as they're spread over a progressively larger number of descendants.
Drachenfels: The eponymous villain has prolonged his life through dark magic for thousands of years, and can recall living through events that are now dismissed as superstitious myths. When he finally dies, his last thought is the realization that, despite this, his entire lifespan is still only an eyeblink in time.
Nicodemus and Tessa are two humans who act as hosts to fallen angels. Tessa (who looks 14-ish) was sold as a slave in ancient Egypt, before she met Nicodemus (who was already immortal and old at that point) and he gave her a coin containing her Fallen Angel. Nicodemus's precise age is unknown, as he makes a point to destroy the church's records of him every few centuries, but he wears the rope Judas used to hang himself around his neck as a tie. The angels that inhabit them are even older. At one point, Lasciel mocks the idea of Harry trying to change her mind, pointing out that if she hasn't in the thousands of years since she fell, he certainly isn't going to make a difference. Additionally, she boasts of living through "infinite thousands" of years, which might as well be true if the White God of the series is the actual creator of the universe.
The Red King is a textbook example. Humanity doesn't know how old he is, because they haven't had a written language that long.
Mab is comfortably older than human civilization (Mother Winter is even older). The construct Archive (not its hosts, who expire) is also somewhere in the region of five millennia old. Then there are the various gods who are at least as old as their religions — so some of them will qualify. We also have the Old Ones, ancient Eldritch Abominations who have been banished from the world in ancient times — but are probably far older than that. In short, The Dresden Files is full of examples.
The current standout example is Archangel Uriel, who claims in Skin Game that he was fighting in wars before Earth had formed, and Word of God is that every angel "predates" linear time.
David Eddings really likes this trope, with it appearing in similar forms in many of his books.
The Disciples of Aldur and Torak in David Eddings's Belgariad and Malloreon. Polgara is the youngest coming in at a "mere" three thousand years old when the series begins. Belgarath himself is the oldest at seven thousand and shows no signs of dying any time soon. They don't generally act it, but they can be very casual in mentioning events that happened centuries previously in ways that other characters find disconcerting. The PrequelBelgarath the Sorcerer really hammers it home when you realise that he's almost as old as human civilization itself.
Even more so the two opposing sentient purposes that drive events throughout the books. And they are actually split personalities formed from the original purpose of the universe which is, oddly enough, as old as the universe itself. The gods fall somewhere in between.
The gods in The Elenium and Tamuli series casually talk about times dating back before humans and trolls had evolved from a neanderthal-esque species. Trolls themselves are apparently immortal, although not really given to remembering anything much about the past. Bhelliom and Klael are older than the planet itself, having created not only it but also many others before.
The Redemption of Althalus also has a similar setup to The Belgariad, with the titular Althalus becoming essentially immortal when he becomes the disciple of a god, and it worked the same for his even older evil counterpart.
The Elric Saga: J'osui C'reln Reyr, the "Creature Doomed to Live" in The Sailor on the Seas of Fate, disobeyed Arioch and was condemned to live until he returned, ten thousand years later.
An Exaltation Of Larks: Mr. Turtle is essentially a time traveler from the end of the universe where entropy has reached maximum and the underlying fabric of reality begins to break down — meaning he's at least a trillion years old. When the protagonist conjures up a number for his age (one hundred billion), Mr. Turtle simply laughs.
Fevre Dream: Damon Julian is old even by vampire standards. It's impossible to say exactly how old, because he was born before vampires had language. At one point, his rival Joshua York argues that Damon isn't really evil, because deep down, part of him is still just a wild animal that can't imagine survival without killing weaker animals.
In Fred, The Vampire Accountant, Fred meets a vampire named Deborah, who is a representative of the Blood Council, which oversees all vampires in the world. When Fred speaks to Lilian about the Blood Council, she tells him that it's composed of some of the oldest and most powerful vampires and is not to be trifled with. Fred suggests that Deborah can't be that old given her name. Lilian counters that the name Deborah is in the Bible, making it at least that old. Several chapters later, Deborah and Fred are sitting in the woods by a campfire, when the former starts talking about how she used to hunt humans before there was such a thing as civilization. Fred realizes that Deborah is incredibly old. She habitually calls any vampire under 1000 years old a child.
Godzilla and Godzilla Raids Again: In Godzilla, Godzilla is stated to be 2 million years old, having survived all that time due to his preternatural longevity. In Godzilla Raids Again the estimate is bumped up to anywhere from 70 to 150 million years ago.
Good Omens: Aziraphale and Crowley are both at least as old as life on earth (both were active in and around the Garden of Eden), although since the timeline of Good Omens is based on the Ussher Chronology, this is less impressive than it otherwise would be.
The Great Ship universe features numerous characters that are tens or hundreds of thousands of years old — the vast majority of humanity are nigh-unkillableTranshumans. Quee Lee, a recurring character, was originally born on Earth eons ago — and was one of the first people to receive the immortality treatments and emergency genes. The short story Alone features a sentient robot which has been walking the hull of the Great Ship for so long that it no longer remembers its name, function, or origin.
The short story "Grotto of the Dancing Deer" by Clifford Simak has an immortal human who was at least twenty-thousand years old. He tells an archeologist friend that he has learned to survive by "being on the fringes, always an observer, never a participant."
The Hero and the Crown: Luthe's teacher Goriolo "could almost remember when the moon was hung in the sky." The heroine's late mother studied with Luthe. For calibration, Luthe remembers the previous active Great Dragon, "one hundred generations ago".
Heralds of Valdemar has Need, an enchanted sword that holds the mind and soul of a mage-smith-priestess from an ancient tribe. She transferred her self into the sword so that she could guide a teenager in rescuing young women from an evil mage. No records remain of the tribe she belonged to, so no one knows how old she is. Even Need herself isn't sure — when she doesn't have a bearer, she has no sense of passing time, and she had no bearer for a long time before the Cataclysm that marked the end of the "Mage Wars" between Urtho and Ma'ar, some 2300 years before the time in which most of the Valdemar novels are set. After she wakes up in the present day, she calls everyone from teenagers to 400-year-old ghosts "youngsters" and has somewhat odd priorities compared to modern folks.
The History of the Galaxy: Several beings and even races have lost count at several billion years old. While this may put them in the Precursor category, they are still around. One novel in particular deals with an Energy Being living in the magnetic fields of a gas giant that members of a Lost Colony call God. A scientist contacts the being and finds out that it is the oldest being in the galaxy and, possibly, the universe. The mindless Forerunners that were originally thought to be the oldest lifeforms in the universe were, in fact, this being's creations meant to carry it from its dying homeworld to other gas giants. The Forerunners also had rudimentary DNA, and many of them ended up dying on planets... starting biological life. So, yes, this being is God in this sense. Others, while younger, include the Evolgs, a race of Energy Beings, and the Emulotti, Human Aliens whom the Evolgs helped become like them to escape a race of Abusive Precursors known as the Shvergs. All this happened billions of years ago, and still-functional Emulotti/Evolg machinery has been found. This kinda puts the other, better known races, who have been around only three million years ago, into the category of "just happened."
In The Restaurant at the End of the Universe, Marvin the Paranoid Android ends up thirty-seven times older than the universe due to incautious use of time travel. It starts when he's stuck on a planet from circa 1980 through the end of time.
"The first ten million years were the worst. And the second ten million, they were the worst, too. The third ten million I didn't enjoy at all. After that, I went into a bit of a decline."
Wowbagger the Infinitely Prolonged made himself immortal by accident and has therefore made it his goal to personally insult every being in the universe, in alphabetical order.
The Host (2008): The Souls have been around for millennia and can live forever. They even spend up to centuries in hibernation when they are traveling in space. At one point, Wanderer, who has lost track of her own age she's been around so long, can only pinpoint an event as being "after the dinosaurs lived on Earth".
In Larklight, it turns out that Art and Myrtle's mother is a member of a race known as Shapers, so-called because they play an important role in creating solar systems.
In The Last Rune series of books some of the Old Gods may have been around for long enough to fit this trope, but the dragons, who predate the creation of the universe, definitely qualify.
The Arisians in the Lensman series. Some of them are two billion years old, and they are capable of plans spanning the entire run of human history.
Treebeard, who's said to be the oldest of all living things, and Tom Bombadil, who claims to be older than the Earth and apparently just showed up all by himself right after the world was made. Lampshaded when Treebeard refers to the ancient wizard Gandalf as "young Master Gandalf", which even given that it's his mortal form that's young is still a bold claim.
Both Ents and Elves are not subject to dying of old age, though Ents get "sleepy" and more and more tree-like as they age, and Elves eventually get "weary" of watching all the death and decay in Middle-Earth and decide to leave it to go to the Undying Lands. Fangorn (Treebeard) is unusual in the fact that he's still active despite being one of the first Ents. Most of the really old Elves in Middle-Earth in Frodo's day have been forbidden to (re-) enter Valinor unless/until they humble themselves and repent.
Elrond has outlived every nation in the world, but he's still a kid compared to Glorfindel (well, kind of) and Galadriel, who is, in some materials, described as being older than thesunnote as in, she can remember watching it rise for the first time ever, though Arda's sun is relatively young (about 7000 years old at the time of the War of the Ring).
CÃrdan, described and only actually seen once, is the oldest Elf around and mentioned at the time of the War of the Ring. He was probably among the first to awaken of the elves at least 11,000 years prior to the War of the Ringnote depending on the conversion factor between Valian years and Solar years, for which Tolkien gave several different values, at the time of the War of the Ring, Cirdan may be over 70,000 (Solar) years old. He's old enough to have a long, white beard. Elves don't grow beards until their third cycle in life, each cycle takes a few millennia.
Gandalf, Radagast, Saruman, and Sauron are actually just 2,000 years old, temporary forms of the Maiar (i.e Angels), all of whom entered Eä (the universe) at the beginning of time. Sauron's particular physical form is older than the others (it was fixed after the Fall of Numenor, over 3,000 years before the War of the Ring), and he had a load of other shapes before that.
The Witch-King is close to 5,000 years old, though he doesn't quite make it. He lasts 4800 years or so.
The Silmarillion: Galadriel is indeed ancient, but one will be astonished when they discover that her parents Finarfin and Eärwen are apparently still alive in Valinor. Finarfin's mother Indis is presumably living there too, as well as Eärwen's father Olwë and his unnamed wife. All of them are at least 12,000 years old. So Galadriel not only has a living mother and father, but three living grandparents. Source material establishes that Elves nevernote Barring divine intervention in one or two cases die permanently from the world, and that their "underworld" is sort of a penalty box.
It's also likely that Fëanor's widow Nerdanel still lives in Valinor, too. She's older than Finarfin and his wife, but still younger than Indis and Olwë.
The Ainur (Valar and Maiar), as well as Ungoliant, are truly ageless, all of them being older than the physical planet. Actually they are older than time itself, which doesn't even have any real meaning to such divine beings.
In The Lorax, the titular creature is as old as time itself.
Malazan Book of the Fallen by Steven Erikson and Ian Cameron Esslemont has a background story going back 300,000 years. And many of the gods and Ascendants were old even back then.
Kallor, the High King, has notched up a hundred thousand years, without the benefit of Ascension even (long story short, he was cursed with immortality but not eternal youth for being such a genocidal bastard).
The T'lan Imass, an extremely important race in the world's history, have each lived for over 300,000 years. They have passed this time hunting down and exterminating all remnants of the Jaghut, who had oppressed them in that distant past. Their racial history goes back into unknowable distances, until 300,000 years before the main story, when nearly all those who still survived took part in a necromantic ritual that made them all undead.
Anomander Rake. He was the first son of the goddess that created existence. (Well, except for the warren of chaos and the eldritch abominations in it.) The Tiste Andii lived for at least thousands of years in their warren before they emerged into the normal world over 300000 years ago. They're so old they don't want to live any more. They just don't care.
The Errant was around by the time The Kharkanas Trilogy prequel takes place and is still alive and kicking during the main series, making him older than many of the current gods of the setting and wittness to the rise and fall of entire civilizations.
The Downstreamers from Stephen Baxter's Manifold series are humans who have survived past the heat death of the Universe, over 10117 years.
In Memory, Sorrow, and Thorn, the Norn Queen is the oldest living thing on the planet, more than twice as old as her closest competition. Said competition is Amerasu, matriarch of the Sithi, who is millennia old and revered as an ancient even by a race of immortals. Exactly how old that makes the Norn Queen probably doesn't bear thinking about.
Master Secundus Minutius Hora, the keeper of time from Momo by Michael Ende. His apparent age fluctuates wildly, but twice he's described as "old, not as an old man, but as a mountain".
Needful Things: Leeland Gaunt has been roaming the Earth for ages, playing his sick mind games with people all throughout history.
The Childlike Empress, who has existed at least as long as Fantastica has (that is, she is as old as human imagination).
The Old Man of Wandering Mountain is just as old, and has existed just as long.
In The Night Land by William Hope Hodgson, the Great Redoubt is an incredibly ancient City in a Pyramid, with a history that "dealt not with odd thousands of years; but with very millions; aye, away back into what they of that Age conceived to be the early days of the earth, when the sun, maybe, still gloomed dully..."
Played for horror in "The Jaunt" by Stephen King, from the short story collection Night Shift. Teleportation works perfectly, as long as you are unconscious. Anyone who goes through it awake describes it as an eternity of disembodied nothingness, which either kills them or drives them insane upon emerging.
"Cautionary Tale" has an alien over ten thousand years old on a fruitless search for immortality.
The Draco Tavern: Several stories deal with this: "The Death Addict"'s danger-seeking alien doesn't have a specific age given, but he's afraid of living long enough to be "the last cluster of protons in the universe". The Chirpsithra have immense lifespans: one in "The Green Marauder" is almost two billion — though relativistic Time Dilation makes her subjective age somewhat less — and visited Earth before its atmosphere had oxygen.
For human characters, Douglas Hooker from "The Ethics of Madness" becomes Known Space's record-holder for this trope, once he's spent 120,000 years (give or take a few centuries) fleeing his nemesis's vessel in a one-man ramship.
The One Who Eats Monsters: Ryn is old enough to remember when humans started using fire, and doesn't bother learning America's name since the country is too young for her to care about. Later in the book we learn she's literally older than Time, as she has existed since before linear time became a thing. With her usual penchant for understatement, she says her years are "difficult to number."
In the Parrish Plessis series, Brilliance is a Homo erectus, making her over a million years old. She was infected with The Corruption, but managed to suppress and harness it to gain eternal life.
Remy Chandler, a.k.a. Seraph Remiel, of the Remy Chandler Novels, remembers, among other things, the War in Heaven, watching Noah as he built the Ark, and a time when humans carried melee weapons around as naturally as modern people carry iPods and cell phones. His memories are interspersed with very recent recollections (less than fifty years at the outside) of his wife, his dog, and his friend, which lends the times when he does remember way back an air of authenticity.
Ann Leckie's The Raven Tower: The Old Gods who came into existence before humanity are the subject of superstitious awe even from other gods. The Strength and Patience of the Hill's earliest memories are of an ocean filled with trilobites, which have been extinct for 250 million years on Earth; after continental drift deposited it on land, it came to enjoy watching the slow revolution of the solar system around the galactic core.
The Inhibitors, a race of genocidal machines dedicated to wiping out all civilizations they deem too advanced, are millions of years old, perhaps even older. Interestingly, their ultimate purpose will not be fulfilled for another three billion years.
In the first novel, the "Sun Stealer" has been around for at least 900,000 years, Body Surfing from person to person.
In The Sack, the titular alien is four hundred thousand years old, and has been the Last of Its Kind for ten thousand of these.
Atoning Unifex in Julian May's Saga of the Exiles and Galactic Milieu is an ancient alien responsible for first setting up the Galactic Milieu. It's eventually revealed that it is actually Mark Remillard, an immortal human who was one of those who travelled back in time to the Pliocene era and ultimately took the long way around to get back to the present, making him around six million years old.
Wax and Wayne: The older kandra (revered as Faceless Immortals carrying out God's will) are often treated this way. They have centuries of experience infiltrating and manipulating human culture, and are in fact older than the world itself. The thing is, the world is only a few hundred years old, since God had to restructure the old, dying world in order to save everyone. So they are technically not examples. Note that the younger kandra are also older than the world (if only by a century or two), but they don't act with the same age because they were mostly confined to their Homeland before the world was changed.
Then there's Hoid. He's been seen in every series in The Cosmere, including ones that were centuries apart, and looks as old as he wants to. In Oathbringer, Shallan asks him if he's a Herald, and when he confirms he's not that, she asks if he's as old as they are. He says that he had lived dozens of lifetimes before they were even born. He was present at the Shattering of Adonalsium, making him roughly the same age as all the original Shards.
Hoid: Some men, as they age, grow kinder. I am not one of those, for I have seen how the cosmere can mistreat the innocent—and that leaves me disinclined toward kindness. Some men, as they age, grow wiser. I am not one of those, for wisdom and I have always been at cross-purposes, and I have yet to learn the tongue in which she speaks. Some men, as they age, grow more cynical. I, fortunately, am not one of those. If I were, the very air would warp around me, sucking in all emotion, leaving only scorn. Other men... other men, as they age, merely grow stranger. I fear that I am one of those. I am the bones of a foreign species left drying on the plain that was once, long ago, a sea. A curiosity, perhaps a reminder, that all has not always been as it is now.
R. Scott Bakker's Second Apocalypse has the Nonmen, a race that was cursed with immortality, which their minds are not equipped to handle. After tens of thousands of years, they tend to suffer from The Fog of Ages and forget most of their lives. Those among them called Erratics can only remember something if it's associated with pain and suffering. They're driven to kill everything they love so that they can lock it in their minds.
In The Secrets of the Immortal Nicholas Flamel, most of the Immortals are centuries old at the very least, but that's nothing compared to the beings that ruled the world before Humans: The Next Generation are thousands of years old, and are still adolescents compared to the Elders, who in turn were young compared to the Great Elders. The Archons are even older, the Ancients were around before them, and the oldest are the Earthlords, who are at least tens of millions of years old.
The oldest humans of all are Gilgamesh and Tsagaglalal, the first that Prometheus awakened. Tsagaglalal seems fine, but Gilgamesh's mind has cracked over thousands of years and he's often left in The Fog of Ages.
The Wall is said to have been built over 8000 years ago.
The weirwoods don't die of old age; some might be around in Westeros from before the Age of Dawn (~12000 years).
However, note that history is unreliable in-universe. Medieval Stasis appears in this world, and there are hints that there's a story-relevant reason for it. So the Wall and weirwoods *may* be as old as we're told, but maybe not.
In John Maddox Roberts' science fiction novel Space Angel, the crew of the eponymous starship encounter a being that lost a battle with another of its kind and wrapped itself in rock to hibernate and regenerate. By the time the Space Angel's crew find it, that rock had evolved into an Earth-like planet, complete with a fully-functioning ecosystem. Although never explicitly stated, it is implied that the being had existed since very near the beginning of the universe.
The Lilim in Stardust are described as being old enough to have noticed continental shift and oceanic movements. At one point, the youngest of them looks out over a petrified forest and recalls when it was a collection of seedlings.
Charles Sheffield's Tomorrow And Tomorrow is about a man who has his wife cryogenically frozen to search for a cure to a nonspecific brain malady. He himself is frozen, after reading up on contemporary music to provide research as a reason to thaw him out. No matter how far into the future he goes, however, his wife's illness is never cured. Eventually most of humanity transcends its former species and leaves him in the dust. His body eventually falls apart from all the freezing and thawing and his brain gets uploaded into a computer. Eons pass, he gets cloned a few billion times, merges with the clones, becomes a god, and restores his wife on his own. The end.
All of the ancient races from the Sequence make a joke out of this trope.
The Monads came before time itself and were, for all intents and purposes, the capital G Gods of the Xeeleeverse; molding froths of virtual particles and turning them into Universes for them to sleep in.
The Xeelee's ancestors first came during the Planck Epoch or around 10^-43 seconds. In layman's terms, they existed before the actual Big Bang itself. 'Modern' Xeelee themselves are only 200 million years younger than the Universe. They created the Antixeelee made of Tachyons to take their seeds back in time to when Baryonic matter becomes stable enough for them to exist. They then spend the next 10 billion years building a great attractor to leave this existence. Some Xeelee have seen the entire universe from the beginning.
The Anti-Matter civilization were at least around during their existential war with the Baryonic Matter federation which created cosmic inflation during the period of 10^-36 to 10^-32 seconds.
The Photino Birds' ancestors, the Xeelee's arch-nemesis, are slightly younger ('younger' in the most loosest sense of the term), first emerging roughly 100 microseconds after the Big Bang. However, the Xeelee only noticed them 200 million years after the birth of the cosmos.
The Quagmites emerged during...well..the Quark Epoch, or around 10^-6 seconds after the Big Bang. By the time the Universe cooled at a comfortable 30 degrees celsius, the Quagmites had already fled the 'dying' Universe 15 million years after the Big Bang.
The Snowmen, the 'youngest' of the ancient races (Again, young being relative here), is estimated to be 10 billion years old, given that this was how old their Snowflake technology were when first discovered by humanity.
A group of humans known as the "Jasofts" — Collaborators with the alien overlords who conquered Earth, who were given eternal life in return.
My name is Luru Parz. I was born in the year AD 5279, as humans once counted time. Now I have lived so long that such dates have no meaning. We have lost the years, lost them in orders of magnitude.
In War of the Dreaming by John C. Wright, this applies to Oberon and Titania, who were around long before humans, and are implied to have been there when life first evolved on earth.
Jack Chalker's Well World has Nathan Brazil, who is as old as they come and then some, pull this one sometimes. He claims to have rebooted the Universe several times. But then, he also admits to being a very talented compulsive liar.
The world of The Wheel of Time is almost impossibly ancient despite its apparent Medieval Stasis as it cycles through ages of prosperity and ruin in a series of recursive triumphs and disasters. To give an impression as to how long this has been going on, while there are your standard ruins and artifacts that are thousands of years old, there are even more mundane structures such as inns, bridges, and warehouses that have been around for ''centuries'' because the denizens of this setting are well-aware of the cycles of their land and build their structures to last.
The last part of The World at the End of Time, set at 10^40 years in the future (see the Literature folder on the Quotes section), has the plasma-based entity Wan-To, who lives inside stars, precariously surviving in proton decay-produced energy and pondering to leave its dead star at its time to go to a black hole where he'll be able to live much longer before that finally decays too.Viktor Sorricaine, the main human protagonist of the novel, is faring much better having lived to those faraway times too thanks to both relativistic time dilation and suspended animation.
Mrs Whatsit gives her age as some absurdly precise number in the billions of years (2,379,152,497 years, 8 months, and 3-days-old exactly). She is still far younger than Mrs Who and Mrs Which.
In Night Watch (Series), there is a vampire named Pyotr who is a Neanderthal, making him at least 35,000 years old. There is also a Cro-Magnon shifter named Hena who turns into a sabertooth. He might be younger than Pyotr, or they might be the same age, since the Neanderthals and the Cro-Magnon coexisted for about 10,000 years.
Dust Devils: Vampire Adam Price is ancient, as he notes and is even noted to possibly being thousands of years old or even older.
, this is less impressive than it otherwise would be.