Jewish leap years are years in which we add an extra month of Adar, and we refer to the two months of Adar in leap years as "Adar Rishon" ("the first Adar") and "Adar Sheni" ("the second Adar"). If a child was born during Adar Rishon of a leap year and the year of his Bar-Misva is a non-leap year, the date of his Bar-Misva is the day in the month of Adar that corresponds to the day of Adar Rishon in which he was born. Thus, if he was born on the twenty-ninth of Adar Rishon, then he will become a Bar-Misva on the twenty-ninth of Adar thirteen years later.
Interestingly, however, this does not apply if the boy was born on the thirtieth of Adar Rishon. The thirtieth day of Adar Rishon marks the first day of Rosh Hodesh Adar Sheni, and we thus consider the child to have been born on the first day of Rosh Hodesh Adar. Hence, thirteen years later, when the year contains only a single Adar, the boy becomes a Bar-Misva on the first day of Rosh Hodesh Adar, or the thirtieth day of Shevat. It thus emerges that if a boy is born on the twenty-ninth day of Adar Rishon during a leap year, and another boy is born a day later, on the thirtieth day, the younger child will become Bar-Misva almost a full month before the older child. The younger child becomes a Bar-Misva on the first day of Rosh Hodesh Adar, whereas the older child becomes a Bar-Misva twenty-nine days later, on the twenty-ninth day of Adar. (See Torat HaMoadim, Laws of Adar)
This Halacha yields a particularly fascinating result in a case of twin boys born just before and after sundown on the twenty-ninth day of Adar Rishon. If the first boy is born before sundown, and the second emerges from the womb after sundown, they of course have different birthdays: the first twin was born on the twenty-ninth day of the month, whereas the second will celebrate his birthday on the thirtieth day. Hence, thirteen years later, if that year is a non-leap year, the younger twin will celebrate his Bar-Misva on the first day of Rosh Hodesh Adar, twenty-nine days before the Bar-Misva of his older brother, which takes place on the twenty-ninth of Adar.
Summary: A boy born during Adar Rishon in a leap year becomes a Bar-Misva on the corresponding day during the month of Adar thirteen years later. If, however, he was born on the thirtieth day of Adar Rishon, then he becomes a Bar-Misva thirteen years later on the thirtieth day of Shevat.
