A boy becomes a Bar Misva, meaning, fully obligated in Torah observance, upon completing his thirteenth year. The question, however, arises as to when precisely he is considered to have completed his thirteenth year. On the one hand, one might argue that a child becomes Bar Misva immediately at nightfall, when the date of his birthday sets in. Thus, for example, if a child was born on the 20th of Nissan, he would become Bar Misva at nightfall on the evening of the 20th of Nissan thirteen years after his birth. Alternatively, however, one might contend that a boy does not become Bar Misva until he reaches the time at which he was born. Meaning, if a boy was born at noon on the 20th of Nissan, then, according to this possibility, he would not become a Bar Misva until noontime on the 20th of Nissan thirteen years later. If so, then he would not count toward a Minyan for Arbit on the night of the 20th of Nissan, and would not able to serve as Hazan, since he has not yet become a Bar Misva.
This issue is subject to debate among the Rishonim (Medieval Halachic scholars). A number of authorities, including the She’iltot, Rabbenu Yisrael Mi’brona, Lehem Hamudot and Meiri, held that a boy becomes Bar Misva only at the time of day at which he was born. The majority opinion, however, maintains that the boy becomes a Bar Misva immediately at nightfall on the evening of his birthday, regardless of what time of day he was born. This is the position taken by the Rama (Rabbi Moshe Isserles of Cracow, 1525-1572), the Mishna Berura (Rabbi Yisrael Meir Kagan of Radin, 1839-1933), the Hid"a (Rav Haim Yosef David Azulai, 1724-1806), Rav Haim Palachi (Turkey, 1788-1869) and the Kaf Ha’haim Sofer (Rav Yaakob Sofer, Baghdad-Israel, 1870-1939). This is, indeed, the accepted Halacha. Therefore, a boy can count toward a Minyan and serve as Hazan on the night of his thirteenth birthday, regardless of when on that date he was born.
Interestingly enough, the work Pardes Yosef suggests drawing proof to this position from the first Mishna in Masechet Berachot (the first tractate in the Talmud), which deals with the time at which one may recite the evening Shema. The Hiddusheh Ha’Rim (Rav Yishak Meir Alter of Ger, Poland, 1799-1866) explains that the Mishna begins with this Misva – the nighttime Shema recitation – because it is the first Misva in which one becomes obligated upon reaching Bar Misva age. This quite obviously assumes that all boys become obligated in Misvot immediately at nightfall on the night of their thirteenth birthday, even if they were born during the day. According to the Hiddusheh Ha’Rim’s insight into the Mishna, then, the Mishna provides proof to the accepted position, that Misva obligation begins at nightfall for all Bar Misva boys.
Summary: A boy becomes obligated in Misvot at nightfall on the evening of his thirteenth birthday, and he can therefore count toward a Minyan and serve as Hazan at Arbit that night.
