Jewish males become Bar Mitzvah ("son of the commandment") and assume adult responsibilities and privileges at age 13, often with a special ceremony in a synagogue, and often with a celebration.
On Saturday morning, March 18, 1922, 12-year-old Judith Kaplan, daughter of Rabbi Mordecai Kaplan, walked to the front of her father’s synagogue in New York City. She recited prayers, read a portion of the Torah in Hebrew and English and shocked "a lot of people," she later recalled, "including my own grandparents and aunts and uncles." This was the first known American Bat Mitzvah ("daughter of the commandment") ceremony.
Except in Italy, before 1922 there was no ritual for girls parallel to a boy's Bar Mitzvah ceremony. The Orthodox Jewish Italian rite for becoming Bat Mitzvah made a great impression on Rabbi Kaplan, who was originally Orthodox, became Conservative, and then founded Reconstructionist Judaism.
Through Kaplan's influence at the Jewish Theological Seminary of America, Jews from all branches of non-Orthodox Judaism learned about and emulated the Bat Mitzvah ceremony. Most Orthodox rabbis strongly rejected it, despite the Italian Orthodox origin.
Judith Kaplan earned degrees in music education from Columbia University, and taught musical education and the history of Jewish music at the Jewish Theological Seminary of America. In 1959, at age 50, she entered the School of Sacred Music of Hebrew Union College, obtained her Ph.D. and taught there until 1979. By the time of her death in 1996, she had composed a significant body of original liturgical music, created a radio series on the history of Jewish music and wrote several books. (Info from Jewish Virtual Library, Wikipedia and other sources.)