In his "History of the Yiddish Language" Max Weinreich regarded the 13th century as a turning point in the history of the Yiddish language. He identifies it as the boundary between the Early Yiddish and Old Yiddish periods. More specifically he writes about a 13th century Babylonian Renaissance characterized by a change in the Ashkenazic norms of Hebrew pronunciation. He says that the change was centered around Rothenburg and involved scholars who bore names that were previously rare or unknown among German Jews but were used by Jews in the Middle East. The name Bablyonian Renaissance comes from Weinreich's beliefs that the pronunciation norms came from Mesopotamia and that the scholars who brought them migrated from there.From Charles Nydorf at Gothic Yiddish.
The post by Mr. Nydorf continues by marshaling evidence that studies of Ashkenazi Jewish genetics corroborate this date as an important one for a major German-Middle Eastern admixture and for a bottleneck followed by a population expansion mostly derived from this core group.