What is the oldest original story by a known author? By original I mean not just a retelling of old myths, so no Gilgamesh. By known I mean we know exactly where and when the author lived, so no Homer. If I had to guess I would say the plays of Aristophanes.
I would submit Aeschylus. His play, The Persians, predates Aristophanes.
Some of the works listed here may have authors of dates known with reasonable confidence. E.g., Sappho.
If by “original” you mean centering on characters or plot pretty much created by the author, I can agree with Aristophanes because Aeschylus’ Persians is based on events contemporary with the playwrite’s life (unique in Greek tragedy), and the Athenians would presumably already “know” the characters in the drama. But i have to admit this is a bit of a hair-splitting argument.
Akkadian cuneiform texts and an Egyptian biographical papyrus include at least some fragments of “original stories”, but I’ve never read them and am not in a position to judge these
One other semi-related tidbit: Petronius’ Satyricon (at least what remains) is arguably the oldest original prose story; one might even call this satire the first novel.