Just Intonation scale is not the oldest scale in the world.
Perhaps the oldest scales on record are the ancient Mesopotamian scales (3500 BC). They were studied by Lou Harrison, as they are present on an old Babylonian cuneiform tablet in the British Museum.
Steven Nachmanvich has now made a Computer programme based on these scales as 'The World of Music Menu' with the help of which, we can actually play and hear these scales.
These scales were later adopted by the Greeks with their own names. (Refer to following table)
Table shows Ancient Mesopotamian and Greek Scales
These scales show that the 12 notes (underlined) in the above table with the addition of S at 1, a total of 13 notes have been known since 3500 BC and all of these also are a part of our 22 Shrutis.