Midrash sobre Êxodo 36:4

(Fol. 69b) Whence do we know that the first generations produced children at eight years? This is to be inferred from the following: (Ex. 35, 30) Bezaleel, the son of Uri, the son of Chur, of the tribe of Judah; and it is written (I Chron. 2, 10-20) And when Azubah, the wife of Caleb, died, Caleb took unto himself Ephrath, who bore unto him Chur. And Chur begot Uri, and Uri begot Bezaleel. And when Bezaleel was engaged in building the Tabernacle, he was at least thirteen years old, as it is written (Ex. 36, 4) Every man from his own work which they were doing; and one is not called a man before the age of thirteen. And there is a Baraitha: The first year Moses prepared all that was necessary for the Tabernacle, and in the second year he erected it and sent the spies. And it is written (Joshua 14, 7) Forty years old was I when Moses the servant of the Lord sent me, etc. Behold, I am this day eighty and five years old. Now, deduct fourteen, the age of Bezaaleel, from the forty of Joshua, when he was sent as a spy, and there remained twenty-six; deduct two years for the three pregnancies with Uri, Chur and Bezaleel, and there remain twentyfour. Hence each of them produced at the age of eight.