Gemara

48b (Rav Yehudah): "Ish Oh Ishah Asher Ya'asu mi'Kol Chatos ha'Adam" equates men and women regarding all punishments;

Just like a man is established to be an adult through one Siman (hairs below), also a woman.

Question: Perhaps either Siman establishes a woman to be an adult!

Rejection: Just like a man is established only through the lower Siman, also a woman.

52a (Mishnah): If a girl (above 12) grew two hairs, she is obligated in all the Mitzvos.

The same applies to a boy (above 13) who grew two hairs.

He can become a Ben Sorer u'Moreh (be killed for stealing from his parents to eat meat and wine) from this time until his beard fills out;

This refers to the lower beard (near the Ervah), but not the upper one. Chachamim use a clean language.

Yevamos 80b (Beraisa): Any male who did not bring two hairs within 20 years is a Seris Chamah, even if he brings hairs later. The (other) Simanim are -- he has no beard, his hair is soft...

(Rav Huna): One must have all the Simanim to be a Seris.

(R. Yochanan): One of them suffices.

When he has two hairs in the beard, all agree that he must have all the Simanim. They argue when he lacks two hairs in the beard.

Chulin 24b (Beraisa): When a man's beard is full, he may be appointed Shali'ach Tzibur and to bless Birkas Kohanim.

Rishonim

Rambam (Hilchos Ishus 2:15): If a boy above 13 years did not bring the Siman below, and he has all the Simanim above, it is a Safek whether he is a minor or adult. If he was not checked below, since he has all the Simanim above, the Chazakah is that he is an adult.

Rebuttal (Ra'avad): He erred. A woman has Simanim above, but a boy does not. The Simanim discussed (regarding a boy) are to be a Seris, and not Simanim of adulthood, until he is 20 years old, even if he has all Simanim of a Seris.

Magid Mishneh: I do not understand the Rambam. The Gemara says that a man does not have an upper Siman. Perhaps the Rambam explains that a man has an upper Siman, just we do not rely on it. I do not know his source for this.

Lechem Mishneh: Since the Rambam holds that we do not rely on the upper Siman, what is the source to rely on it if we did not check the lower Siman? Meforshim ask why we rely on the upper Siman in a woman, since the verse equates her to a man. Tosfos (48a DH Ba) says that if the upper Siman came, presumably she is an adult. We say that the lower Siman already came. However, the Hekesh to a man should teach that it does not help! The Rambam answers that the upper Siman helps only when we did not check the lower Siman, both for men and women.

Or Some'ach: Perush ha'Mishnayos explicitly says that there is no upper Siman in a man. He retracted in the Mishneh Torah. He learned from Kidushin 81b, which says that breast and hair growth is a Siman 'in this and this', i.e. in a boy and girl. Rashi explains that it means 'in both versions', i.e. whether we discuss a girl of nine years or 12 years. The Rosh (Kidushin 4:25) refuted this, for the same words appear in Berachos 24a, and there the other version discusses a girl of three years. See the Rambam in Hilchos Kri'as Shema (3:19) and Hilchos Edus (9:8). The Rambam explains that in Nidah, we say that there is no upper Siman in a man, i.e. the Torah wrote only the lower Siman. I.e. we expound "Ki Yazid", which alludes to the ability to father a child, which depends on the lower Siman.

Note: This is difficult. In Halachah 7, the Rambam said that breasts are a Siman in a girl. I did not find other Meforshim ask a contradiction between the Gemara in Nidah, and Kidushin and Berachos. Most men never develop breasts. Does Version #2 in Kidushin permit them to sleep naked with their mothers forever?! Seemingly, 'in this and this' can refer to a boy and girl for what pertains to them, i.e. breasts and hair growth in girls, but only hairs in boys. One can explain the Rosh and Rambam in all places in this way. (Here, 'Simanim l'Ma'aleh' in the Rambam could mean the Simanei Seris he gave in the previous Halachos.) However, R. Mano'ach in Hilchos Kri'as Shma connotes like the Or Some'ach.

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

Rosh (Megilah 3:15): In Chulin, we require that his beard filled out to being appointed bi'Kvi'us (on a fixed basis). To do so b'Akrai (occasionally), two hairs suffice.

Tosfos (Yevamos 80b DH d'Hevi): The Ri says that two hairs in the beard do not count like bringing two hairs of adulthood, even though two hairs anywhere else on the body count. He can have all the Simanei Seris, for two hairs does not count like having a beard, until his beard filled out.

Poskim

Shulchan Aruch (OC 53:8): If one does not have a beard, even if he reached the age for his beard to fill out, it is called that his beard filled out. Therefore, if he is 20, even though he does not have a beard, we may appoint him.

Mishnah Berurah (27): According to the Gra's text in Maseches Sofrim, this Heter (for a 20 year-old without any beard) depends on the Heter in Sa'if 9 (for a Seris to be Shali'ach Tzibur).

Kaf ha'Chayim (50): This is only if he has two hairs. If not, he is a minor until the majority of his years (i.e. past 35 years).

Rema: Similarly, if he has a beard, even a little, this is called his beard filled out if he is 18 or above.

Beis Yosef (DH v'Heicha): If he is big in years, even if he does not have a beard, perhaps we can appoint him to be Shali'ach Tzibur. As long as it is recognized that he reached the age for his beard to fill out, it is called that his beard filled out. The Rambam connotes that lack of a beard disqualifies only if this is due to his youth, but if he is old enough, even if he does not have a beard at all, he may Duchan. It is considered as if his beard filled out. Maseches Sofrim (14:17) supports this. It says that one must be 20 years and have a beard to be Shali'ach Tzibur or Duchan, but not if he is 20 and has no beard, even if he looks like a Seris. (If he is a Seris, he is an adult.) Some permit even a Seris. If his beard sprouted, he is permitted even from 18 years. The Nimukei Yosef says in the name of Ge'onim that if he has only a little beard, since he is old enough, it is as if his beard filled out.

Shulchan Aruch (128:34): 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.

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.

Shulchan Aruch (OC 39:1): If a minor wrote Tefilin, they are Pesulim.

Magen Avraham (1): He must be a proper adult, i.e. he brought two hairs after 13 years. If it is a Safek (whether he brought two hairs), he may not write until he is 18, like it says in EH 148:25 and CM 35 regarding testimony.

Bi'ur Halachah (DH Oh): All the Acharonim argue with the Magen Avraham, who is concerned lest he is a minor until 18. However, we cannot disqualify what he wrote (if he is above 18 without a beard), since the Chayei Adam and Shulchan Shlomo are lenient l'Chatchilah. The Pri Megadim brought from the Rema (53:8) that if he is 18, even if he has a small amount of beard hairs, it is called that his beard filled out. However, perhaps there (to be Shali'ach Tzibur) he has two hairs, and the beard is merely for Kevod Tzibur. Here, to establish him to be an adult, perhaps we need a full beard. Having hairs in a few places in the beard is not enough (Tosfos Yevamos 80b). That Tosfos is the source that if his beard filled out, we need not check for (lower) hairs.

Mishnah Berurah (2): Since this is mid'Oraisa, he is Pasul until his beard fills out, i.e. there are many hairs, even if they are very small, or until he lives most of his years.

Shulchan Aruch (CM 35:1): A child may not testify until he brings two hairs after 13 full years. If his beard filled out, he need not be checked.

Beis Yosef (DH v'Im): The Tur learns from Tosfos that if his beard filled out, he need not be checked. Why did he say here Tzamach (sprouting)? It seems that there it is a stringency of Chalitzah to require a full beard. However, what is his source to distinguish? The Rambam (Hilchos Edus 9:8) says that Simanei Bagrus above in a man suffice, like he said in Hilchos Ishus. The Ra'avad challenged this, and the Magid Mishneh was unable to defend the Rambam.

SMA (4): I say that there is no difference between sprouting and filling out. The Gemara merely means that two hairs do not suffice. 'Filled out' refers to many hairs, but they need not be long, just like hairs below need not be long.

Bach (4): Tosfos wrote that hairs in the beard do not establish adulthood! The Tur holds like Avi ha'Ezri, brought in the Mordechai, that a recognizable beard suffices. In Hilchos Chalitzah, the Tur was stringent like the Tosfos and Rosh (Yevamos 8:7), to require a full beard l'Chatchilah. This is because we find that Ge'onim were more stringent than about the Beis Din for Chalitzah than for monetary cases, therefore the Tur was more stringent about Chalitzah.

Gra (5): Tosfos connotes that hairs in the beard do not establish adulthood! Rather, the Tur learns from the Rambam. They challenged the Rambam from Nidah 48b, which says that a man has only the lower Siman, I answer that in a man, surely we rely on one Siman alone, for Ben Sorer u'Moreh is only for three months after two hairs, and the upper beard comes much later. We cannot say that a man's lower Siman always comes before his upper Siman, for Yevamos 80b connotes that the upper can come first. In any case, the lower Siman usually comes first, like in a woman. We see this! The Rambam relies on the upper Siman only if we did not check the lower Siman.