Actually, historians recognice today as the oldest writing (!) the so-called 'plant-like' and 'trident-shaped' markings, which have been found on fragments of pottery dating back 5500 years. This is in Pakistan, at a site called Harappa, in the region where the great Harappan or Indus civilisation flourished.
Before that (they were found in 1999), it was suggested that the oldest writing might have come from Egypt.
Clay tablets containing primitive words were uncovered in southern Egypt at the tomb of a king named Scorpion.
They were carbon-dated to 3300-3200 BC. This is about the same time, or slightly earlier, to the primitive writing developed by the Sumerians of the Mesopotamian civilisation around 3100 BC.
Since the Harappan language died out and did not form the basis of other languages, we probably will never know what is/was written on there: