Gemara

(Beraisa): When a man's beard is full, he is proper to be Shali'ach Tzibur, be Over Lifnei ha'Teivah (go in front of the Bimah or Aron Kodesh) and bless Birkas Kohanim.

Megilah 24a (Mishnah): A child may read the Torah and translate, but he may not be Pores Al Shma (say Birkos Keri'as Shma aloud for the Tzibur), be Over Lifnei ha'Teivah, or give Birkas Kohanim.

Rosh Hashanah 29a (Mishnah): Anyone exempt from a matter may not exempt the Rabim.

Rishonim

Rambam (Hilchos Tefilah 8:13): If one's beard did not fill out, even if he is a great Chacham, he may not be Shali'ach Tzibur, due to Kavod Tzibur. He may be Pores Al Shma from when he brings two hairs after 13 years.

Question: In Chulin, we say that when a man's beard is full, he may be Shali'ach Tzibur, be Over Lifnei ha'Teivah, and bless Birkas Kohanim. Our Mishnah says that a minor may not be Over Lifnei ha'Teivah, or give Birkas Kohanim. This implies that if be brought two hairs, he may!

Answer #1 (Rosh Megilah 3:15): In Chulin, we require that his beard filled out. That is to being appointed on a fixed basis. Here we discuss doing so b'Akrai (occasionally).

Answer #2 Rashba (Chulin 24b DH Nismalei): Here, the Gemara says that it is improper for the Tzibur to appoint him to be fixed Shali'ach Tzibur, due to Kavod Tzibur. He may give Birkas Kohanim or be Shali'ach Tzibur b'Akrai without appointment. This is why here the Gemara said 'he is proper to be Shali'ach Tzibur (from when his beard is full). It did not say 'he is Kosher' or 'he may be Shali'ach Tzibur.' There (in Megilah), it does not say 'it is improper', rather, 'he may not give Birkas Kohanim until he brings two hairs.' Rav Amram Gaon says that R. Natrunai Ga'on answered that it is the ideal Mitzvah not to be Shali'ach Tzibur or give Birkas Kohanim until his beard fills out. When possible, a fully mature man is better than a youth. Surely, if the only way to say Kedushah and Amen Yehei Shmei Raba is to allow a boy 13 years and a day to be Shali'ach Tzibur, we do so. Anyone exempt from a matter may not exempt the Rabim. This is because he is not yet obligated in Mitzvos. If one is 13 years old and a day, when it is impossible (to find someone else), he may do so.

Ran (Chulin 5a DH Garsinan): The Gemara did not say only 'go in front of the Teivah' because only to be appointed fixed Shali'ach Tzibur requires a full beard. B'Akrai, when there is no one else (qualified), any adult may be Shali'ach Tzibur. A proof is from Megilah 24a. The Mishnah says that a minor may not be Over Lifnei ha'Teivah or give Birkas Kohanim. This implies that any adult may descend, i.e. b'Akrai. R. Natrunai Ga'on says so.

Ran and Rashba (ibid.): Rashi explains that he may be appointed to be Shali'ach Tzibur for all needs of the Tzibur, e.g. to blow the Shofar, excommunicate, and to appoint him Parnes (leader) and go in front of the Teivah to pray. I disagree. Being a Parnes and other needs of the Tzibur is called Chazan ha'Keneses, and not Shali'ach Tzibur. Rather, the Gemara teaches one matter. He may be appointed Shali'ach Tzibur to go in front of the Teivah.

Teshuvas ha'Rosh (4:17): If one's beard did not fill out, he may pray (be Shali'ach Tzibur) b'Akrai, like one of the residents who prays when it crosses his mind, but the Tzibur may not appoint him, and the Shali'ach Tzibur may not appoint him to lighten his load and pray in his stead at times.

Teshuvas ha'Rosh (4:22): A 13-year old may be pray (for the Tzibur) b'Akrai, but he may not be appointed a fixed Shali'ach Tzibur until his beard filled out. Even if Chazan ha'Keneses (who is older) prays on Shabbos and days of gathering (perhaps this refers to Monday and Thursday, when they read the Torah - PF), a youth whose beard did not fill out may not be Shali'ach Tzibur during the (rest of the) week. This is called Kavu'a, even though also someone else is appointed. After bringing two hairs, he may be Shali'ach Tzibur only b'Akrai, without appointment.

Poskim

Shulchan Aruch (OC 53:6): We do not appoint a Shali'ach Tzibur if his beard did not fill out, due to Kavod Tzibur.

Beis Yosef (DH uv'Seder): According to the Rambam, the Tzibur may pardon their honor (and let him be Shali'ach Tzibur). The Tur learned from Teshuvas ha'Rosh (4:17) that even if there is an appointed Shali'ach Tzibur, he may not appoint a youth (whose beard did not fill out) to pray for him at known times, even if the Tzibur pardons their honor.

Taz (2): This is astounding. Surely the Rambam agrees that the Tzibur may not pardon their honor. Here, their honor is Hash-m's honor. They honor Hash-m through a Shali'ach Tzibur proper to advocate on behalf of the Tzibur. This is clear from what the Rambam (Perush ha'Mishnayos) wrote in Gitin (60a), that we do not read Keri'as ha'Torah from a (lone) Chumash (rather, only from a full Sefer Torah), due to Kavod Tzibur. In Siman 143:2, the Poskim did not mention the reason of Kavod Tzibur. If pardon helps, they should have mentioned the reason, to teach that pardon helps! Rather, pardon does not help for matters that pertain to Kavod Shamayim.

Magen Avraham (9): 'Due to Kavod Tzibur' connotes that the Tzibur can pardon their honor. However, the Shulchan Aruch forbids the Tzibur to appoint him. This implies that their pardon does not help. In any case, l'Chatchilah one should not do so. The Shulchan Aruch (579:1) connotes that for a Ta'anis, we require a real beard (i.e. an old person), unlike Tosfos in Megilah (24a DH v'Eino), who says that it suffices if it filled out.

Mishnah Berurah (23): The Tzibur cannot pardon their honor.

Mishnah Berurah (24): One whose beard did not fill out may not be Shali'ach Tzibur in a Ta'anis, Rosh Hashanah or Yom Kipur, even if the Tzibur pardons their honor.

Beis Yosef (DH v'Chasav): The Kolbo says that he should not be Shali'ach Tzibur until his lower (pubic) beard fills out. It seems that he holds that this is what the Gemara discusses. The other Poskim did not specify. This connotes they explain that it is the upper beard.

Shulchan Aruch (ibid.): However, from when he brings two hairs (after 13 years) he may be Shali'ach Tzibur occasionally.

Beis Yosef (DH v'Ein): The Kolbo (11:8:3, citing R. Noson) says that a man may not be Shali'ach Tzibur even b'Akrai until bringing two hairs, and the hairs grow enough to bend the end to the beginning. Less than this is not an "Ish". We require an Ish, for it says "Madu'a Basi v'Ein Ish."

Mishnah Berurah (25): The Magen Avraham (10) says that if he is 13 years old, we need not check for hairs. For mid'Rabanan matters, we rely on Rava's Chazakah (that he brought two hairs immediately). The Pri Megadim says that according to the Rambam, who holds that Tefilah is mid'Oraisa, such a Shali'ach Tzibur may not be Motzi the Rabim unless we are sure that he brought two hairs.

Bi'ur Halachah (DH Aval): The Ran and Ramban permit only when there is no one else. The other Rishonim did not mention this. It seems that the Shulchan Aruch permits in any case, for when there is no one else, the Shulchan Aruch (Sa'if 7) permits appointing him to be the Shali'ach Tzibur, i.e. even Kavu'a (Levush). Ma'amar Mordechai permits occasionally even when others could be Shali'ach Tzibur. Really, the proof from Sa'if 7 is not solid. Perhaps that is when we know that no one else will be available, but Stam we do not appoint one without a beard, for perhaps someone with a beard will be available.

Kaf ha'Chayim (40): One who may not be Shali'ach Tzibur due to an Aveirah is forbidden even occasionally, until he does Teshuvah.

Shulchan Aruch (ibid.): This is only if the Tzibur does not appoint him, and the Shali'ach Tzibur does not appoint him to lighten his load through being Shali'ach Tzibur at known times.

Beis Yosef (DH veha'Tosfos): Tosfos in Chulin (24b DH Nismalei) says that if his beard filled out, he can be a fixed Shali'ach Tzibur to pray specifically on fasts and Ma'amados. This connotes that at other times, he can be Shali'ach Tzibur even if his beard did not fill out. However, in Sukah (42b DH ha'Yode'a), Tosfos says that Chulin discusses being Shali'ach Tzibur Kavu'a and (i.e. or) to pray on Yom Kipur, fasts and Ma'amados. This is like the Rosh, Rashba and Ran. The Tur and Meforshim that I brought hold that a minor cannot be Shali'ach Tzibur even b'Akrai.

Rebuttal (Bach 2 DH Kasav): The Beis Yosef understood that Tosfos in Chulin requires a full beard only to be a fixed Shali'ach Tzibur for fasts and Ma'amados. This is wrong. Tosfos requires this to be a fixed Shali'ach Tzibur, or to pray even once for fasts and Ma'amados. He erred because his text of Tosfos said 'Lehispalel'. Really, it is u'Lehispalel, with a Vov.

Note: Our text is 'Oh Lehispalel.'

Taz (4): The Tzibur may appoint one without a beard to be prepared to pray if sometimes the Chazan is not in the Beis ha'Keneses. This is not called Kavu'a, for perhaps it will never occur. It is forbidden only if there is a fixed time that will definitely come.

Mishnah Berurah (26): The Bach and Eliyahu Rabah say that even this is Kavu'a. B'Akrai is only if he was not appointed at all.

Beis Yosef (DH umi'Divrei): Why is there a custom that a minor is Shali'ach Tzibur for Ma'ariv on Motza'ei Shabbos? Perhaps Chachamim were particular only about Shacharis, which contains Kedushah in Birkas Yotzer Or and Shemoneh Esre, and the Shali'ach Tzibur must repeat Shemoneh Esre to be Motzi the Rabim. Since a minor is not obligated, he cannot be Motzi the Rabim. I heard that the Avudraham opposed the practice, and R. Yitzchak Di Le'on agreed to abolish it. The Rashba (1:239, citing the Ra'avad) says that Berachos and Tefilos are mid'Rabanan, and a child who reached Chinuch is obligated mid'Rabanan, so he could be Motzi people. The reason the Mishnah forbids a minor to be Shali'ach Tzibur is due to Kavod Tzibur. It would be disgraceful for the Tzibur. Based on this, we can justify the custom, and say that the Tzibur pardon their honor. Even though Rashi explained that the reason is because one who is not obligated cannot be Motzi others, we can say that Ma'ariv is different, for it is optional (Berachos 27b).

Bi'ur Halachah (DH Yuchal): The Pri Megadim is stringent even if the youth is an Avel (mourner). I say that we may be lenient about Ma'ariv to make him a fixed Shali'ach Tzibur, for we do not repeat Shemoneh Esre at Ma'ariv. Also, presumably the Tzibur pardons their honor for an Avel, so we can join the opinion of the Beis Yosef and Shiltei ha'Giborim who are lenient in such a case.