If a child is born in Adar of a non-leap year, and he/she reaches the age of 13/12 in a leap year, when is it appropriate to celebrate the child becoming bar/bat mitzvah? Would that be Adar I or Adar II?
A boy born in the month of Adar of a non-leap year celebrates his bar mitzvah in the second Adar, if his bar mitzvah year is a leap year. (Rama on Code of Jewish Law, Orach Chaim 55:10) Nevertheless, there are some halachic authorities that maintain that the bar mitzvah should be celebrated in the first Adar. In order to satisfy this opinion, the boy should put on tefillin starting from the date of his birthday in the first Adar. (Shevet Halevi 6:9.)
The same applies to Bat Mitva. It's celebrated on the second adar.