Answer:
   
   A bar/bat mitzvah s considered legally responsible to fulfill the
   mitzvos. A boy becomes bar mitzvah at the age of thirteen and one day.
   Girls become bat mitzvah at age 12. This is usually celebrated by the
   child being called to read from the torah at the shabbat closest to
   their bar/bat mitzvah. As is common in Judaism, there is often a party
   afterwards, which can vary from the simple home reception to an overly
   ornate "theme" celebration.
   
   Until the child reaches bar/bat mitzvah age, they are responsible only
   as part of chinuch (training). After the bar/bat mitzvah, the child is
   legally an adult in the eyes of Judaism. This means the following:
     * They are now counted for a minyon (prayer quorum of ten).
     * They are responsible for wearing tefillin.
     * They are eligible for aliyot (being called up to read the Torah).
     * They are responsible to fast on fast days.
     * They are responsible for observing the mitzvot.
       
   With respect to Bat Mitzvah. The event itself has been recognized for
   many generations:
     * Rabbi Yosef Chaim in his book "Ben Ish Chai" [1883-1909, a rabbi
       from Bagdad] talks about the day of a girls Bat Mitzvah as a day
       of celebration on which she should wear a new outfit and say
       "She'he'chiyanu" and include her entrance to the "burden of
       Mitzvot" (Ol Mitzvot).
     * Rabbi Yitzchak Nissim Z"l quotes from Rabbi Mussafya (1606-1675,
       born in spain a rabbi and personal doctor of King Critian the IV
       of Denmark. later he moved to amsterdam) that the day of the Bat
       Mitzvah is a day of celebration and the dinner is a "Se'udat
       Mitzvah" (mitzvah dinner).
     * In Italy (Torrino and Milan) it was customary to gather the Bat
       Mitzvah girls and the community during a weekday, the girls stood
       in front of the open Aron Kodesh and recited (dividing the prayers
       among them) a special prayer written for them which included a
       blessing of Shehechiyanu and ended "Baruch Ata Hashem Lamdeynee
       Chukecha" (bless ..teach me your laws). [note, the prayer was also
       said by bar mitzvah boys]. Then the rabbi speaks and blesses the
       girls and their families. Afterwards, there is a Se'udat Mitzva at
       the girls' home
       
   However, none of these ceremonies involved the girl reading from the
   Torah. The first public bat mitzvah ceremony in which a girl read from
   the Torah is believed to have been for Judith Kaplan Eisenstein z"l,
   the daughter of Rabbi Mordecai Kaplan, the founder of the
   Reconstructionist movement. The ceremony has since been adopted by
   almost all movements in Judaism.
   
   Note that the focus of the bar/bat mitzvah should be the actual
   ceremony, not the party afterwards.
   
   More information on Bar and Bat Mitzvah may be found at
   [5]http://www.barmitzva.org/