The statement that “Lehi … having been taught in the language of the Egyptians therefore he could read” the engravings on the brass plates of Laban quite clearly indicates these plates were written in the Egyptian language. Thus they were almost certainly not started until after the flood and the tower of Babel, as there was no Egyptian language before those events. The brass plates were probably not started until after the Israelites went down into Egypt in the days of Joseph, although the writers on these plates may have had access to records that had been written earlier.
Two other evidences supporting this thesis are: (1) Laban “was a descendant of Joseph, wherefore he and his fathers had kept the records” (1 Nephi 5:16), and (2) the great prophecies “of Joseph, who was carried into Egypt … are written upon the plates of brass” (2 Nephi 4:1-2), as these records contained “the five books of Moses” (1 Nephi 5:11). Other writers continued recording on these plates “even down to the commencement of the reign of Zedekiah, king of Judah” (1 Nephi 5:12)—the very year Lehi left Jerusalem (1 Nephi 1:4).