What makes someone Bar & Bat Mitzvah?

When Jewihs children turn 12 or 13 they celebrate their Bar Mitzvah or Bat Mitzvah.  What is a Bar Mitzvah or Bat Mitzvah? Why does it happen at 12 or 13 years old? What does the ceremony entail? What needs to be done to become a Bar Mitzvah or Bat Mitzvah? Learn about the Bar Mitzvah and the Bat Mitzvah, its traditions and its true meaning in Judaism.