"Eldest, that's what I am...Tom remembers the first raindrop and the first acorn...he knew the dark under the stars when it was fearless — before the Dark Lord came from Outside..."
― Tom Bombadil (LOTR)
"Stop. I have learned much from you. Thank you, my teachers, and now for your education. Before there was time, before there was anything, there was nothing, and before there was nothing, there were monsters. Here's your gold star!"
― Sweet P/The Lich (Adventure Time)
"I lived ten thousand lifetimes before the first of your kind crawled out of the mud."
― Vaatu (Avatar: The Legend of Korra)
"This is one little planet, one tiny solar system, in a galaxy that's barely out of its diapers. I'm old, Dean, very old, so I invite you to contemplate how insignificant I find you."
Juno, Minerva, and Jupiter (Assassin's Creed) are members of the First Civilization, beings who not only predate the human race, but created it.
Ulthane (Darksiders) is one of the Makers, a race that predates both Heaven and Hell.
Davoth, the Dark Lord of Hell (Doom), was the first of the Father's Primevals and is believed to be over a decillion years old.
Jack of Blades (Fables) has existed for thousands of years, and was already ancient by the time humanity came into existence.
The Ancients (God of War) are a race of elemental golems said to be the lingering fragments of the primordial being and progenitor of the Jötnar, Ymir, making them as old as the beginning of time itself.
The Precursors (Halo) were a race of beings believed to be even older than the universe itself.
The Elder God (Legacy of Kain) predates recorded history and claims to be the "engine of Life" itself.
Chip/Light Gaia (Sonic the Hedgehog) is a primordial deity who embodies day, light, and rebirth...
...while his counterpart, Dark Gaia, is a primordial energy being that symbolizes darkness, night, and destruction.
Cybele (Valkyrie Crusade) is a primordial goddess that predates concepts such as life and death, being the one that created life itself in the first place.
Eden Maker (Valkyrie Crusade) is a primordial goddess and one of the creators of the Celestial Realm.
Birthday (Valkyrie Crusade) as the embodiment of the celestial realm's birthday, is as old as the celestial realm itself.
Gilgamesh's (Valkyrie Crusade) age is virtually immeasurable, as she is eons years old.
Illyria (Angel) is one of the Old Ones, an ancient race of pureblood demons who lived during the Primordium Age.
The First Evil (Buffy the Vampire Slayer) came into existence long before the universe was created, and will remain long after its end.
The Archangels (Supernatural) predate the universe itself, battling The Darkness alongside God.
Eve (Supernatural) predates the angels themselves, and is obscurely related to the Leviathans.
The Leviathans (Supernatural) are the very first beasts ever created by God, and are as old as time, predating the angels, humans and the souls itself.
The Shadow (Supernatural) is a primordial being whose existence predates all things, including Amara, God, Death, Creation and Destruction, as he is the original emptiness/nonexistence they all eventually emerged from.
Dendar, The Night Serpent (Forgotten Realms) is the original primordial nightmare and as such is as old as time and conscious thought itself.
Known as the First and the Fatherless, Tom Bombadil (The Lord of the Rings) is the oldest being in all of Middle-earth and beyond, having already existed long before even the creation and awakening of the Ainur by Eru Illuvatar.
Nyx/Nox (Greco-Roman Mythology) the goddess of the night, one of the primordial gods (protogenoi) who emerged at the dawn of creation.
Like all Obyriths, Pale Night (Planescape) has been around long before the multiverse ever existed, as she and her kind have been destroying countless realities outside of the proper multiverse before it was even born.
The Fae (SCP Foundation) are the ancient, original race that ruled the world and reality long before the Children of the Night/SCP-1000 and the Children of the Sun ever even existed. Even predating the concept and existence of Adam and Eve.
Eru Ilúvatar (The Lord of The Rings) is the The All Powerful creator of the Tolkien universe.