Niddah 46 - When Does One Become Bat- or Bar-Mitzvah
A boy becomes an adult (bar mitzvah - son of the mitzvah) at thirteen years and one day, while a girl becomes a bat mitzvah (daughter of the mitzvah) at twelve and one day - these are the words of Rabbi Yehudah the Prince. However, Rabbi Shimon ben Elazar holds just the opposite: a girl at thirteen, and a boy at twelve. What are their respective proofs?
The reason of Rabbi Yehudah, " God built the rib that he took from the man into a woman ..." The word "built", "vayiven" is connected to "binah" - understanding, to tell us that women are inherently more intelligent and reach the level of mental maturity earlier than men do. And Rabbi Shimon, how does he explain this word? He says, "built" means that God braided Eve's hair, and indeed in cities by the sea they called a braided coiffure a "building."
In addition to being thirteen, the boy, to be considered a bar mitzvah, must have grown two pubic hairs. If he grows them before he is thirteen, they are not considered as anything, and are presumed to have grown from a mole in his skin. If he grows them after he is thirteen, then he is a full-fledged adult. If it is not known when the hairs grew, then - if we don't use the view that if he grew them before thirteen, they count even now - his real status is in doubt. However, since a majority of boys do grow two hairs after they become thirteen and one day, we can presume that the boy in question is the same. This majority-determined status is sufficient for most laws, save for the some exceptional cases.
Art: Albert Anker - Portrait of a Young Boy