According to Jewish tradition, the age of thirteen for a boy and the age of twelve for a girl is the age of responsibility. In order to symbolize the attainment of the age majority for a young person, it became customary in the mid-1800s for a boy to be called to read a passage from the Torah on the Saturday immediately before his birthday, and for a girl to read a section of the Haftarah.
Reform Judaism equalized the ceremonies so that both male and female are asked to fulfill the same ritual requirements and to conduct the same portions of the service. Indeed, throughout its history, the B'nei Mitzvah celebration was always seen as a major community event. Therefore, the entire congregation is always encouraged to attend and every B'nei Mitzvah service is a congregational event.