Hwæt we Gar-Dena     in gear-dagum
þeod-cyninga     þrym gefrunon,
hu ða æþelingas     ellen fremedon.

... and so on. Eighth century AD, earliest, from linguistic evidence; though the substance of the story is probably older. Arthur, if he existed, was probably too busy fighting the Danes to take a great deal of interest in their (admittedly impressive) heroic myths.