philosopher,
You are absolutely wrong about girls not doing extra mitzvot once they turn 12. On that day girls become chayev in everything from eating matzah and retelling the story of the Exodus on Pesach (not to mention maror, Hallel, and four cups of wine), to making kiddush on Shabat and Yom Tov, to birkat hamazon, to hearing the Megillah, to lighting Shabat, Yom Tov, and Chanukah candles, to taking Challah, to prayer. The number of Torah mitzvot for which men are chayev and women are patur is quite small.