You need to be more specific Tali. Do you mean marks made by pigment....this might include traces of red ochre on artefacts made 10,000 years ago - or do you (as I suspect) mean painting as in thing in a frame that hangs on a wall?
You might find that the oldest "painting" is part of an illuminated manuscript, such as the Book of Kells ( written and illustrated about the year 800 ) If a book made out of parchment and illustrated with mediaeval colours and inks is kept almost always closed, it could last for many centuries. Some mediaeval manuscripts of 1,000 years old look just as if their pictures were finished yesterday.