The concept that manhood begins at age 13 is deduced by the Rabbis from the story of Dinah in Gen. 34, where it says (v.25)
that Dinah's brothers Simeon and Levi took "each man his sword" and attacked the city where Dinah was a captive. At that
time, Levi was just 13, so we see that a boy of 13 is called a man. In Judaism, a boy becomes responsible for observing all
the commandments when he reaches the age of 13; he is therefore called "bar mitzvah" -- "subject to commandment". At
that point he is no longer legally a minor and is personally accountable for his deeds. However, according to the Talmud
(Shabbos 89b), Divine punishment for a boy's sins is postponed until he reaches the age of 20, possibly to give him a chance
to repent and reform.
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