Gemara

11b (Beraisa): A case occurred in which R. Yehoshua bar Chananyah went to help lock gates.

R. Yochanan ben Gudgoda: Refrain! You are a singer, and not a gatekeeper!

Suggestion: R. Yochanan holds that a singer who locks gates (alone) is Chayav Misah, so Chachamim decreed that he may not help.

13b (Mishnah): The only time a minor may enter the Azarah for Avodah is when the Leviyim sing;

R. Eliezer ben Yakov says, they do not count towards the 12 required Leviyim. They were called Tze'irei (those who pain) ha'Leviyim.

Megilah 24a (Mishnah): A minor may not be Over Lifnei ha'Teivah (Shali'ach Tzibur) or give Birkas Kohanim.

Chulin 24b (Beraisa): When a man's beard is full, he may be appointed to exempt the congregation in prayer and to bless Birkas Kohanim.

Sukah 42a (Beraisa): If a Katan knows how to Duchan (give Birkas Kohanim), we give to him a share of Terumah in the granary.

Kidushin 71a (R. Tarfon): Once, I followed my uncle to the Duchan, tilted my ear, and I heard the Kohen Gadol say it (Hash-m's 42-letter name) amidst the melody of the other Kohanim.

Rishonim

Rambam (Hilchos Tefilah 15:4): An adolescent Kohen may not Duchan until his beard fills out.

Question: In Sukah, we say that if a minor knows how to Duchan, we give to him a share of Terumah in the granary. Elsewhere, we say that a minor may not Duchan!

Answer #1 (Rashi 42a DH Cholkin): The 'Katan' brought two hairs (after 13 years, i.e. he is Bar Mitzvah).

Rebuttal (Tosfos Chulin 24b DH Nismalei): The Gemara connotes otherwise, for the same Beraisa taught that if a Katan knows how to slaughter, one may eat from it, and we establish this to be when an adult supervised!

Answer #2 (Rosh, Megilah 3:15, and Tosfos ibid.): He may Duchan with other Kohanim, like it says in Kidushin, 'once, I followed my uncle... They were called Tze'irei ha'Leviyim.

Hagahas R. Perla (2): The reference should be to Erchin 13b. The words 'once I followed...' do not belong here. They are not in Tosfos.

Korban Nesan'el (400): (His text of the Rosh said 'in Erchin.) This ('once, I followed...') is not in Erchin. Perhaps the Rosh brings a proof from the case in which R. Yehoshua went to help lock gates. Even though one may not lock alone (if this is not his task), he may help.

Tosfos (ibid.): A minor may Duchan with adults, like we find regarding the Shir of Leviyim in Erchin. He may not Duchan by himself until he brings two hairs.

Ran (Chulin 5a DH Garsinan): Tosfos answered that he may Duchan with adults to harmonize the voices, just like they did for Shir, like it says in Erchin. A minor may not Duchan by himself.

Answer #3 Rashba (24b DH Nismalei): In Sukah, the minor does not Duchan, rather, he knows how to Duchan, i.e. he knows the laws. He is called a Kohen who serves, therefore, he receives Terumah. However, he may not Duchan until he brings two hairs.

Question: In Chulin, we say that when a man's beard is full, he may be Shali'ach Tzibur and bless Birkas Kohanim. In Megilah, we say that a minor may not do these. 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 bi'Kvi'us (on a fixed basis). Here we discuss doing so b'Akrai (occasionally).

Answer #2 (Rashba and Tosfos ibid.): Here, the Gemara says that it is improper for the Tzibur to appoint him to be Shali'ach Tzibur bi'Kvi'us, 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 the ideal Mitzvah is 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. When it is not possible, a youth is fine.

Mordechai (Megilah 815): A Kohen's beard must fill out to Duchan regularly. Even before this, he may Duchan occasionally to establish (publicize) that he is a Kohen. I found written that a bachelor may not Duchan, for one who dwells without a wife is without Simchah. One who blesses should be b'Simchah. We find that Yitzchak blessed after he ate, and it says "va'Yvarachu Es ha'Melech... Semechim v'Tovei Lev" (Melachim 1:8:66).

Tosfos (Chulin 24b DH Nismalei): Megilah 24a connotes that once a boy is Bar Mitzvah, he may Duchan.

Poskim

Shulchan Aruch (OC 128:34): A minor who did not bring two hairs may not Duchan by himself at all.

Beis Yosef (DH Katan): Before a Kohen bring two hairs, he may Duchan only with adults. After he bring two hairs until his beard fills out, he may Duchan even by himself, but not bi'Kvi'us. Why did the Tur equate these two categories, and say that he may not Duchan alone? Indeed, the Rambam holds like this. He says that an adolescent Kohen may not Duchan until his beard fills out. He must discuss when he is alone, for even a minor may bless with others, like it says in Sukah. Also Semag says so. Even so, the Tur should have brought the Rosh's opinion! Perhaps his text of the Rosh was unlike ours.

Bach (20): I say that the Tur relied on what he wrote in Siman 53, that a Shali'ach Tzibur must be proper and regularly read Torah, Nevi'im and Kesuvim, but one (who does not fulfill this) may be Shali'ach Tzibur b'Akrai. However, one may not appoint him bi'Kvi'us until his beard fills out. All the more so, he may Duchan as long as he is Kosher, even if he is not meticulous about Mitzvos and everyone murmurs about him. Therefore, the Tur did not elaborate here (to permit b'Akrai). The Shulchan Aruch rules like this.

Taz (31): The Tur lived in Ashkenaz, where the custom is to Duchan only on Yom Tov, therefore everything was b'Akrai, so it is permitted with other Kohanim. This seems primary.

Magen Avraham (48): Even if he is 13 years old, one must ensure that he has two hairs to avoid a Berachah Levatalah.

Eliyahu Rabah (58): Why is it a Berachah Levatalah? A minor blesses on all Mitzvos, even though he is not commanded! Rather, if he is 13 we rely on the majority, who bring hairs (immediately), like in Simanim 53:5, 55:5 and 199:10 (regarding being a Shali'ach Tzibur, and joining to make a Minyan and Zimun).

Mishnah Berurah (121): If he is 13 years old, we assume that he brought two hairs.

Kaf ha'Chayim (194): Machatzis ha'Shekel says that the Magen Avraham discusses when we know that he did not bring two hairs. The Magen Avraham connotes unlike this. The custom is like the Eliyahu Rabah.

Mishnah Berurah (122): He may not Duchan even b'Akrai, for it is not honorable for the Tzibur to need a minor's Berachah.

Shulchan Aruch (ibid.): He may Duchan with adults in order to learn and practice.

Magen Avraham (49): He may bless (...Asher Kidshanu... v'Tzivanu Levarech...)

Mishnah Berurah (123): This is unlike Chinuch for other Mitzvos, which is from the age of five or six. Rather, it is when he knows how to Duchan like the custom of Kohanim.

Bi'ur Halachah (DH Aval): Olas Tamid says that he is permitted with adults even Keva. Eliyahu Rabah connotes like this. The Pri Megadim was not convinced. The Mechaber concluded 'then he may Duchan even alone.' This connotes like Olas Tamid, and so ruled Chayei Adam.

Shulchan Aruch (ibid.): One who brought two hairs may Duchan even by himself, however, only b'Akrai, but not bi'Kvi'us, until his beard fills out. Then, he may Duchan even by himself bi'Kvi'us.

Beis Yosef (DH u'Kevar): Kolbo says that we discuss the lower (pubic) beard filling out. It seems that the Poskim disagree.

Magen Avraham (50): Now that we Duchan only on Yom Tov, even before his beard filled out he is permitted every Yom Tov to establish that he is a Kohen. Keva is only every day.

Mishnah Berurah (124): Also this is due to honor of the Tzibur.

Bi'ur Halachah (DH u'Mihu): Olas Tamid says that if he is the only Kohen and he is 13 but he did not bring hairs, he may Duchan even Keva, like we say about a Shali'ach Tzibur. Eliyahu Rabah disagrees. There we are lenient, lest the Tzibur lose Kaddish, Kedushah and Borchu. The Eshkol connotes like Olas Tamid. A midget without a beard should not Duchan alone, even if his city is used to him, lest one from another city think that a minor may Duchan alone (Pri Chodosh, from the Yerushalmi).

Kaf ha'Chayim (200): Beis Meir (17) says that since all the (other) Poskim omitted this, this shows that they hold that the Bavli disagrees. Any adult may Duchan by himself.

Bi'ur Halachah (DH v'Lo): Tosfos (Sukah 42b DH ha'Yode'a) says that on Yom Kipur and fast days when we request much mercy, even b'Akrai is considered Keva. It is astounding that the Acharonim did not bring this. In practice, this requires investigation.

Kaf ha'Chayim (199): Eshel Avraham (50) rules like Tosfos. (We hold like this.)

Shulchan Aruch (ibid.): If one is old enough to have a full beard, even though it is not full, it is called that his beard filled out. See above 53:8.

Mishnah Berurah (126): There we say that if he is 18, even if he has a little beard, this is called his beard filled out. Before this, we require a truly full beard.