According to some, the first documented Bar Mitzvah celebration is
referred to in the Torah: "And the child [Isaac] grew and was weaned,
and Abraham made a great feast on the day that Isaac was weaned"
(Genesis 21:8). According to one opinion expressed in the Midrash,
this was the day that Isaac turned thirteen; the day when he was
"weaned" from his childish nature, and assumed the responsibilities of
a Jewish adult. In Jewish literature, this verse is often used as a
source for the celebration made in honor of a boy's acceptance of the
mitzvot at age thirteen.
Also, see: http://www.ajrsem.org/teachings/journal/5765journal/krieger5765/
The Zohar1 relates how on the day of his son's Bar Mitzvah, Rabbi
Shimon bar Yochai made a feast which was akin to a wedding
My research suggests that the celebratory Bar Mitzvah feast became a
unanimously accepted Jewish custom some four hundred years ago.
As for the cause of the celebration, this is the day when a Jewish
person is given the obligation and resulting privilege of observing
G‑d's commandments. One would be hard-pressed to think of a more
joyous occasion to celebrate together with friends and family!
Rabbi Shlomo Luria, noted 16th century Polish scholar, rules that the
Bar Mitzvah feast is a seudat mitzvah, a "mitzvah repast," which means
that participating in this meal is actually a mitzvah. FOOTNOTES
Zohar Chadash, Genesis 10c; 15d.
and from the wikipedia page you link:
Many sources indicate that the ceremonial observation of a bar mitzvah developed in the Middle Ages,
Simon Ẓemaḥ Duran, in his "Magen Abot" to the Baraita, quotes a Midrash interpreting the Hebrew word (= "this") in Isa. xliii. 21—"This people have I formed for myself, they shall pronounce [A. V. "set forth"] my praise"—as referring by its numerical value to those that have reached the age of thirteen. This seems to imply that at the time the Midrash was composed the Bar Miẓwah publicly pronounced a benediction on the occasion of his entrance upon maturity. This is confirmed by the Midrash Hashkem (see Grünhut's "Sefer ha-Liḳḳuṭim," i. 3a): "The heathen when he begets a son consecrates him to idolatrous practises; the Israelite has his son circumcised and the rite of 'pidyon ha-ben' performed; and as soon as he becomes of age he brings him into the synagogue and school ('bet ha-keneset' and 'bet ha-midrash'),in order that henceforth he may praise the name of God, reciting the 'Bareku' (Benediction) preceding the reading from the Law." Masseket Soferim xviii. 5 is even more explicit: "In Jerusalem they are accustomed to initiate their children to fast on the Atonement Day, a year or two before their maturity; and then, when the age has arrived, to bring the Bar Miẓwah before the priest or elder for blessing, encouragement, and prayer, that he may be granted a portion in the Law and in the doing of good works. Whosoever is of superiority in the town is expected to pray for him as he bows down to him to receive his blessing."