Question

How do you know that Sumerian society developed after the Stone Age???

Answer 1

There aren't any stoneage artifacts (?)
not so sure but I think that's the answer anyways.

Answer 2

Sumerians developed writing and left written records and literature, had a complex society, and used metal tools.

http://history-world.org/sumeria.htm

http://www.theology.edu/sumer.htm