The month of Adar is unique among the months of the Hebrew calendar in that an extra month of Adar is added during a leap year. According to the Hebrew calendar system, an extra month of Adar is added twice every seven years, such that in these years the calendar features Adar Rishon (the "first" Adar) and Adar Sheni (the "second" Adar).
This system gives rise to several interesting situations regarding the Bar Mitzva of a boy born during the month of Adar. First, if a boy is born during the month of Adar on a regular year, containing only a single Adar, but the year of his Bar Mitzva is a leap year, and thus features two Adars, his Bar Mitzva would occur during Adar Sheni. Thus, for example, if a boy was born on the fifteenth of Adar and the year of his Bar Mitzva happens to be a leap year, he would become a Bar Mitzva on the fifteenth day of Adar Sheni.
If a child was born on Adar Rishon of a leap year, and the year of his Bar Mitzva is likewise a leap year, then he would of course become a Bar Mitzva on the same day in Adar Rishon.
Finally, if a child was born on the thirtieth day of Adar Rishon during a leap year, and the year of his Bar Mitzva is not a leap year, then since the thirtieth of Adar Rishon is also the first day of Rosh Chodesh Adar Sheni, he becomes a Bar Mitzva on the thirtieth day of Shevat, which is also the first day of Rosh Chodesh Adar. His Bar Mitzva would occur not on the thirtieth day of Adar, but rather on the thirtieth day of the previous month, Shevat, which also marks the first day of Rosh Chodesh Adar.
