The Mishna in Pirkeh Abot (5:25) teaches, "Ben Shelosh Esreh Le'misvot" – a youngster becomes obligated in Misvot upon reaching the age of thirteen. At this point, he may be counted toward a Minyan and may serve as Hazzan. The source for this rule is "Halacha Le'Moshe Mi'Sinai" – an oral tradition taught to Moshe at Mount Sinai. The Gemara in Masechet Sukka (5b) teaches that all Shiurim – halachic measurements – were taught as a "Halacha Le'Moshe Mi'Sinai," and this includes the "measurement" of adulthood, when a boy becomes obligated in Misvot. Rashi, however, in his commentary to Abot, finds a Biblical source for this rule. The Torah uses the word "Ish" ("man") in reference to Shimon and Levi when they waged war on the city of Shechem ("Ish Harbo" – Bereshit 34:25), and, as Rashi shows, Levi – the younger of these two brothers – was thirteen years old at this time. This establishes that a boy attains the status of "Ish" – a man – at the age of thirteen. The Maharil (Rav Yaakov Moelin, Germany, d. 1427) refutes this proof, noting that the use of the word "Ish" in this context does not necessarily mean that this word would not be used if Levi was younger. Therefore, the Maharil concludes that there is no textual basis for this rule, and it was transmitted through oral tradition. Some suggested an allusion to this Halacha in a verse in the Book of Yeshayahu (43:21) in which Hashem pronounces, "Am Zu Yasarti Li, Tehilati Yesaperu" – "I have created this nation for Me, that they tell My praise." The word "Zu" in Gematria equals 13 (7+6), thus hinting to the fact that it is at this age when Hashem wants us to praise Him and perform Misvot. There is a debate among the early authorities as to when precisely a boy is considered a Bar-Misva. The She'iltot (Rav Ahai Gaon, d. 752) writes that a boy becomes a Bar-Misva the moment he fully completes his thirteenth year – meaning, at the time of day when he was born thirteen years earlier. Thus, for example, according to this opinion, a boy who was born at 2pm cannot be counted for a Minyan or serve as Hazan on his thirteenth birthday until 2pm, the point at which he has completed thirteen full years. The consensus among the Poskim, however, is that a boy becomes Bar-Misva once the date of his thirteenth birthday arrives, in the evening. This is, indeed, the Halacha. Therefore, regardless of the time of day of a child's birth, he may serve as Hazan already at Arbit on the night of his thirteenth birthday. The Yalkut Yosef writes that the thirteen years are counted from the child's birth even if he was born prematurely and needed to spend a significant amount of time in an incubator. In addition to the requirement of completing thirteen years, a boy must also have reached a certain point of physical maturity to be considered a Halachic adult. Specifically, he must have grown two pubic hairs. The Rama (Rav Moshe Isserles, Cracow, 1530-1572), based on a ruling of Rav Yosef Kolon (1426-1490), writes that a child who has turned thirteen may be allowed to serve as Hazzan on the assumption that he has reached the point of physical maturity. This assumption may be relied upon with respect to matters instituted by the Sages (as opposed to Torah obligations), and thus, since praying with a Minyan is a Misva ordained by Sages, a child who reached Bar Misva age may lead the service. The Ribash (Rav Yishak Bar Sheshet, Algiers, 1326-1408) went even further, allowing relying on this assumption even with respect to Torah obligations. According to his view, a full-fledged adult may fulfill his Torah obligation of Kiddush on Friday night by listening to Kiddush recited by a boy who has just turned thirteen, on the assumption that he has reached physical maturity. Hacham Ovadia Yosef ruled that those who wish to rely on this position may be allowed to do so. However, Hacham Ovadia's son, Hacham David Yosef, writes in Halacha Berura that one must not assume a boy's physical maturity with respect to Torah obligations such as the Friday night Kiddush, and this assumption may be made only with respect to Rabbinic obligations. All opinions agree that a thirteen-year-old boy may read the Megilla in the synagogue on Purim, since the obligation of Megilla reading was instituted by the Rabbis. If it is known that a young man has not yet reached this stage of physical development, then he is not considered a Bar-Misva even though his thirteenth birthday has passed. In fact, even if he is older than thirteen, he is not considered a Bar-Misva if it has been determined that he does not have the physical properties required to establish halachic adulthood. If, Heaven forbid, a man does not physically develop until the age of 35, at that point he is considered a "Saris" – an adult man who will never experience physical maturity, and he may thus be counted toward a Minyan. Until then, however, he cannot be considered an adult and may thus not be counted toward a Minyan.