I'd assert it is before they (or someone observing them) did write down their history. Writing goes back quite a ways, but the idea of writing down histories only goes back as far as Herodotus. Also, even if the histories are lost or only alluded to in later derivative works, the time period covered might still be considered historic (though sometimes considered traditional, legendary or semi-historic).

Pre-Roman Britain would generally be considered prehistoric, (Chalcolithic based on the Cornish sources of tin, or early Iron Age, but well past neo-lithic), mainly due to the lack of written histories. Not to say they didn't have oral histories, just that there is no record available for historical study.