Wait guys, the oldest evidences of slings that I know are some clay bullets from Southern Italy (Neolithic, approx. 5500-5000 years bp), and some much older cave pictures in France; I've to look at my books but if you can wait this evening, I'll give you further info.
We already discussed about this and you will find better info in that thread, look for it using the search button
Greetings,
Mauro.