What precisely do you mean by "artefact" and "in use"?

That's a poorly phrased question. At least without robust definitions of “artefact” “in use” and “historical”.

Any precise answer is going to be fairly uninformative. “this artefact exists and is still in use” doesn't tell us very much. In any case the answer is almost certainly some unnoticed bit of reused building material.

Still it seems popular and we've had a few wrong answers deleted (mostly guesses less than a thousand years old) so lets look at what the options are.

The earliest period from which have artefacts is the palaeolithic. Artefacts from that period aren't that common so while someone could pick up one of the stone scrappers from the period and use it to scrape flesh from bone I doubt anyone does. In any case I doubt such one off use is what is being asked about.

The next date of interest is about 200,000 years before present. This is about the point when anatomically modern humans appear. With them we see the first classes of artefacts that could be argued to still being used as originally intended in the form of graves and certain hand axes. We've found hand axes of apparently impractical sizes (I've seen some earlier dates for some of these but I'm sceptical). Its possible that some of these were made to be looked at as a cool thing and some of them are on display in museums. Other interpretations are however possible.

The oldest fairly unambiguous artworks are around 30-40K years before present. If they were meant to serve only as artworks then they are still in use (on display in various museums).

Skipping over the Mesolithic (which for the purpose of the question can be considered late palaeolithic only more so) we come to the neolithic. If there isn't some neolithic stonework reused in an in use structure somewhere I'd be rather surprised. A case can also be made for religious artefacts still being in use. Avebury likely originally had a relgious function and there is enough modern religious use that it has to be organised via a rota.

More broadly the rise of argiculture during the neolithic allows for the possibility of suriving small walls, pools and dams and drainage ditches.

Large scale stuff starts to bring the problem of if it is the same artefact. Bits of the grand canal in china date back to the 5th century BC but have been highly modified. Bits of the chinese dujiangyan irrigation system dating from around 250BC are pretty original. However that also brings up to the date of the oldest buddist temples.

By the time you get to the Romans a lot of stuff in terms of dams, canals and bridges are still in use. Engraved gems and cameos are sometimes still worn in rings. Roman samian ware (recovered from shipwrecks) was still in use in the 19th century.

It would depend on what you mean by "in use". The Great Pyramid of Giza and Stonehenge are both 4-5,000 years old and are still in use as tourist attractions, if you count that.