The paper reviews briefly the relevant views on the age of forty as found in the Talmud and in the Arab tradition, views which are the background of the later development in the Middle Ages. Afterwards the philosophical discussions on the age of forty found in the writings of Moses ibn Ezra, Jehuda ibn Abbas, Shem Tov Falaquera, Levi ben Abraham, Nissim of Marseilles, Prat Maimon and Isaac Aboab are analyzed. The views of authors like Falaquera and ibn Abbas might have influenced the first kabbalistic restrictions against divulging secrets to students who had not yet reached the age of forty. Such restrictions occur in a work of R. Moses ben Simeon of Burgos and in the school of Abraham Abulafia. Special emphasis on the interdiction against revealing certain kabbalistic secrets can be found in the works of R. Shem Tov ibn Gaon who related it to his teacher R. Solomon ibn Adret. A number of kabbalists of the fourteenth, fifteenth and sixteenth centuries knew about this interdiction and it was influential also among pupils of R. Isaac Luria. Two last important occurrences of the subject discussed above appear in a document dictated by a court of the rabbis of Frankfurt to R. Moses Hayyim Luzzatto and signed by him, during the polemics against the Frankists.

The appendix deals with some kabbalistic commentaries on the talmudic dictum “Restrain your children from higgayon” (Berakhot 28b).