A WAG, 15,500 years ago. Now you?
What's a WAG?
Your date would be when the last ice age was still in place. The Adam of Genesis lived in an Eden watered by 4 rivers but where rain was unknown--this is consistent with an ice age.
I've seen speculation that the Sphinx is older than that, maybe 30,000 B.C. but most archaelogists won't buy that. If so, though, it would be just about the only evidence we have of a pre-Adamic civilization.