I've heard that one shouldn't study Kabbalah unless one is over forty years old? Is this true?

The great Kabbalist R. Isaac Luria (1534-1572), began the study of Kabbalah at the age of seventeen and died at the age of thirty-eight! His equally famous contemporary R. Moses Cordovero (1522-1570) began at the age of twenty. Many other famous Kabbalists also began the study early.

This prohibition has come from Ashkenazic (East European) Jews and has never applied to Sepharidic (Middle Eastern) Jews. The historical basis for the "rule" comes from opponents of Kabbalah within Judaism who (successfully) attempted to restrict its study. At the root of this was the heresy of false messiah Shabbatai Tzevi (17th. C) which resulted in large numbers of Jews leaving the orthodox fold. This heresy had deep Kabbalistic underpinnings, and in the attempt to stamp out Shabbateanism, Kabbalah itself became suspect, and specific prohibitions against the study of Kabbalah were enacted (e.g. the excommunication of the Frankists in Poland in 1756).

A further factor was the degeneration (in the eyes of their rationalist opponents) of 18th. century Hasidism, which had roots both in Kabbalah and Shabbateanism, into "wonder working" and superstition. The rationalist faction in Judaism triumphed, and the study of Kabbalah became largely discredited, to the extent that many Jewish publications written earlier in this century discuss Kabbalah (if at all) in a very negative way.

Greg Burton has supplied this (mildly amusing) post from America OnLine, from a Rabbi Ariel Bar-Zadok:

" One thing I assure you, I am not a "new ager", nor am I sympathetic to anything that is not pure, authoritative Kabbalah. Remember, Kabbalah means "to receive". I am an Orthodox Sephardic Rabbi, ordained in Jerusalem. I teach only from the true texts, many of which most Rabbis for whatever reasons have never read. I document all my sources so as to verify to you that these teachings are authentic. (I must also admit that I have studied other religious and meditative systems, in this way I feel comfortable and confident to discuss them). My classes are open to all, Jew and Benei Noah alike, men and women, (in accordance to Tana D'vei Eliyahu, Eliyahu Raba, Chapter 9). By the way, according to the Chief Rabbi of Israel, Rabi Ovadiah Yosef (Yehaveh Da'at 4,47) quoting Rabbi Moshe Cordovero, one only has to be 20 years old to study Kabbala, and not 40. THIS IS THE HALAKHA!!"

This still leaves R.Isaac Luria looking embarrassed, but R. Moses Cordevero scrapes in under the bar