It is understandable why the medieval Talmudic scholars of southern France and their outstanding leaders prohibited anyone under the age of thirty from reading philosophy and metaphysics: they perceived the dangers to the young unfortified minds of falling into heresy or, worse, into atheism. As for the actual practice of mystical exercises, other European rabbis limited it to those who were over forty because of the mental perils, particularly madness, involved in it. The Godhead, "The Most Hidden of the Hidden" in the Hebrew phrase, is utterly beyond human reach.
-- Notebooks Category 20: What Is Philosophy? > Chapter 2: Its Contemporary Influence > # 261