I'm not the one assuming anything. I know construction of those things often took about a life time or two.
All we can do is date the cuts of the blocks and they all belong to a civilization older than sumer. Somehow sumer is the oldest civilization according to official history books.
They are not the oldest exactly. Just one of the older ones we know more about because of how much of their writings survived. We think human civilisation started about 12000 years ago
The oldest megaliths are dated at 15k BC. But it's true we don't know much about them simply because everything have been utterly destroyed.
