"Bar Mitzvah” literally means “son of the commandment.” “Bar” is “son” in Aramaic, which used to be the vernacular of the Jewish people. “Mitzvah” is “commandment” in both Hebrew and Aramaic. “Bat” means daughter in Hebrew and Aramaic. A girl becomes a bat mitzvah at the age of 12 according to Orthodox and Conservative Jews, and at the age of 13 according to Reform Jews. Simply put, boys start observing the commandments after becoming a bar mitzvah. The same happens to girls.

Many people think these two terms mean events, but bar mitzvah and bat mitzvah mean persons. Children under the age of 13 are not obligated to observe the commandments in Jewish culture. Anyway, kids are encouraged to do as much as possible to learn more about the obligations they will have as grown-ups.

The bar mitzvah ceremony is known all over the world as one of the symbols of the Jewish culture. Anyway, it is not required, and the custom of having a big party varies from one community to another. So, to become a bar mitzvah, first of all, a boy needs to reach the age of 13. The party is optional.

More Info: en.wikipedia.org