When a person attains adult status according to halacha, and becomes obligated in mitzvot. For boys, it is at age 13 ("bar mitzvah"); for girls, age 12 ("bat/bas mitzvah").
Many people have a celebration and ceremony to mark the occasion. Bar mitzvah boys usually get their first aliyah latorah (called up to read from the Torah) at their bar mitzvah celebration.
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