How does one select the parasha of the Bar Mitzvah? By the week the kid is born? In the Jewish calendar? Does it vary around the world?
It is customary for the boy on his first Shabbat after his 13th Hebrew birthday to have an Aliya in shule. Many barmitzvah boys read the Parasha or the Haphtarah for that occasion. The Parasha is not necessarily the one that he was born on; it has to be checked in the calendar of his 13th Hebrew birthday. Most times it will be the same torah reading all over the world, in seldom cases where the reading in Israel and the Diaspora in different, the good calendars should pick those dates up.