Bar mitzvah is a Jewish coming of age ritual for boys, whereas bat mitzvah is the equivalent for girls.
According to Jewish law, when a Jewish boy is 13 years old, he becomes accountable for his actions and becomes a bar mitzvah.

A girl becomes a bat mitzvah at the age of 12 according to Orthodox and Conservative Jews, and at the age of 13 for Reform Jews.

Before the child reaches bar mitzvah age, parents hold the responsibility for their child’s actions. After this age, the boys and girls bear their own responsibility for Jewish ritual law, tradition, and ethics, and are able to participate in all areas of Jewish community life.