Gemara

48a (Mishnah - Chachamim): If the upper Siman (breasts that lean) came, she may do Yibum or Chalitzah, for the lower Siman (two hairs) can come before the upper Siman, but not vice-versa.

49b (Beraisa): Simanim of girls are always checked by women.

R. Yehudah permits women to check before and after the Perek (the twelfth year), but not during the Perek itself. A girl may not rely on checking of women to (do Mi'un or Chalitzah and) marry;

R. Shimon says, women may check even during the Perek itself.

A woman is believed to be stringent, to say that a girl is an adult and may not do Mi'un, or that she is a minor and may not do Chalitzah yet;

She is not believed to be lenient, to say that a girl is a minor to allow Mi'un, or to say that she is an adult to allow Chalitzah.

Question: Why does R. Yehudah say that women check her even after the Perek?

(Rava): We need not check a girl above 12. She is Muchzekes to be an adult.

Answer: Rava's Chazakah forbids Mi'un. We must check her to allow Chalitzah.

Question: Who taught 'a woman is believed to be stringent, but not to be lenient'?

Answer #1: R. Yehudah taught it. He refers to checking during the Perek.

Answer #2: R. Shimon taught it. He refers to checking after the Perek. He argues with Rava.

Rishonim

Rosh (Seder Chalitzah, in Yevamos 12:19): The judges must know that the Yavam is at least 13 years and one day, and the Yevamah is at least 12 years and one day. Women check her. If we see that she has large breasts, or his beard filled out, we need not check her or him for hairs.

Tosfos (49a DH v'Iba'is): If a minor Yavam or Yevamah comes in front of us for Chalitzah, l'Chatchilah it is good to check for hairs before the Perek (years of adulthood), and again afterwards. If they have the same hairs, we wait for new hairs. If they came after reaching the Perek, it is a proper Chalitzah if they have hairs. We are not concerned lest the hairs were there before the Perek. If she has large breasts, one need not check for hairs, for we say that the upper Siman cannot come before the lower Siman.

Mordechai (58): Sefer ha'Terumah discusses men checking the upper Siman, and women checking if she has many long hairs. I am unsure whether this is precise. I heard from Maharam that women should check the upper Siman.

Darchei Moshe (EH 169:9): Why should women be believed about many long hairs to establish her to be an adult even if we are unsure about her years? The Mordechai says that women are believed due to the Chazakah that she brought Simanim right after 12 years. The Gemara and the Tur support this. We conclude that R. Shimon does not rely on women, because he disagrees with Rava's Chazakah. If so, when we do not have this Chazakah, and we are unsure whether she is 12 years old, we cannot believe women. Perhaps it is because it is a Milsa d'Avida l'Igluyei (a matter prone to become known), which people do not lie about.

Note: I do not understand why this is Avida l'Igluyei. Normally no one would look there! Further, hairs could grow later, or she could cut them off!

Beis Shmuel (6): Also the Bach brings in the name of Semak that men must check the upper Siman, unless we know that she is above 12. Then, women may check, for there is a Chazakah that she brought Simanim right after 12 years. The Darchei Moshe said that we rely on women because it is Avida l'Igluyei. Women are not believed when we are unsure whether she is 12 years old. We do not say that it is Avida l'Igluyei, because two hairs is not so prone to become known. This is why Sefer ha'Terumah says that nowadays we do not do Mi'un, for we are not experts to check the entire body for hairs. This is difficult, for also the upper Siman is Avida l'Igluyei!

Suggestion: The upper Siman is no better than two hairs. The upper Siman helps because it cannot come before hairs. Therefore, we do not rely on women for the upper Siman when we are unsure about her years.

Rebuttal (Beis Shmuel): If so, it would not help to check through men (for even if they see the upper Siman, perhaps she is not 12 years old)! Rather, we must say that the Darchei Moshe answered only according to the Mordechai, who holds that we check the upper Siman through women. According to Sefer ha'Terumah, who holds that women do not help for the upper Siman, it is not clear why we rely on women regarding many hairs.

Beis Meir (DH v'Lulei): I would say unlike the Darchei Moshe, that the Mordechai understood that Sefer ha'Terumah discusses when we do not know her years, for if we did, two hairs would suffice. The Mordechai asked based on what he said above (57), that women are believed about two hairs due to Rava's Chazakah. Even though Rav Yehudah said that the hairs must come out of pores, and we are not expert about pores, if there are many hairs or long hairs, one need not check more. Also, we may assume that she passed 12 years. If the upper Siman came, Vadai she is an adult. This is like the Bach. (We are not concerned about pores if there are many hairs or long hairs.) The Beis Shmuel disagrees, and perhaps indeed, the Tur and Shulchan Aruch are unlike the Bach, but Sefer ha'Terumah is like the Bach. There is no disgrace to check the upper Siman, and it is good to do so, for it is hard to check for two hairs, since we are not expert about pores. Therefore, we can say that we know her years. Even so, even men would not be believed about two minimal size hairs, for we are not expert about pores. Seder Chalitzah of Maharam Padovah says that women can check for much hair if we know her years.

Tosfos (Kidushin 63b DH Bnei): If a girl is tall like an adult, even if she has only two hairs, we establish her to be an adult even if we do not know her years.

Tosfos (Yevamos 80b DH d'Hevi): The Ri says that bringing two hairs is not considered a beard, unless his beard filled out.

Poskim

Shulchan Aruch (EH 169:10): The judges must know that the Yavam is at least 13 years and one day, and the Yevamah is at least 12 years and one day, and that they brought two hairs after reaching this age.

Beis Yosef (DH Kasvu): Tosfos (49a DH v'Iba'is) says that l'Chatchilah, we should check a minor Yavam or Yevamah for hairs before the Perek, and again afterwards. If they have the same hairs, we wait for new hairs. If they came after they reached the Perek, it is a proper Chalitzah if they have hairs. We are not concerned that the hairs were there before the Perek.

Gra (27): This is like we say that there is a Chazakah that witnesses do not sign a document (unless they know that the parties are adults). All the more so, Beis Din (must ensure that they are adults).

Shulchan Aruch (ibid.): If the Yevamah has large breasts, one need not check for hairs.

Beis Yosef (DH u'Mah she'Chasav v'Im): The Rosh, Rashba, Sefer ha'Terumah and Tosfos say that if she has the upper Siman, we need not check for the lower Siman.

Gra (32): We cannot say that the upper Siman is Shuma (not a real Siman), for it does not come before its time.

Be'er Heitev (12): If the breasts merely lean, we must check also her years. If they are as big as they are normally in a woman who reached her full height, we need not check her years, even if she did not reach her full height (Masas Binyamin 10).

Shulchan Aruch (ibid.): Similarly, if the Yavam's beard filled out, one need not check for hairs.

Teshuvas R. Akiva Eiger (1:7): The Magen Avraham (Reish Siman 39) says that we rely on Rava's Chazakah for mid'Rabanan laws, e.g. to be Yotzei Tefilah through a youth, but not for Torah laws, like we find regarding Chalitzah. However, perhaps this is only when there is Chezkas Isur, but one may be Yotzei Birkas ha'Mazon or Kidush through a youth. Perhaps Birkas ha'Mazon is like Chalitzah, for the person was Vadai obligated to say it. However, if so, we should need to check also for Tefilah! The Rif (34a) holds that the Pesul of Chalitzah of a minor girl is only mid'Rabanan. If she did not do Chalitzah again after maturing, the Chalitzah helped. (The Beis Shmuel (169:49) holds that the Rambam (Hilchos Yibum 4:16) agrees.) If so, also for mid'Rabanan laws we do not rely on the Chazakah! Do not say that Chalitzah is different, for its source is mid'Oraisa. We are lenient about Chalitzah, and say that b'Di'eved Chalitzah of a minor helped. If a minor was Motzi an adult in Tefilah, he was not Yotzei even b'Di'eved, and he must pray again. If a minor Yavam did Chalitzah, it is Pasul mid'Oraisa, so two witnesses must check him. We must be concerned for the Rif and Rambam who hold that Chalitzah of a minor Yavam is Pasul only mid'Rabanan, so we cannot learn from a Yevamah to a Yavam. Chalitzah is called Devar sheb'Ervah (which requires two witnesses), like the Nimukei Yosef says (Yevamos 34a DH Beino). One should be stringent, since it is easy to check a boy with two witnesses. There are reasons to distinguish boys from girls. It is disgraceful to check girls through men, so automatically even one woman is believed. Proper testimony is needed for a boy.

Pischei Teshuvah (12): However, we can say that this is only for checking two hairs. Regarding checking for much hair, which is Avida l'Igluyei, even R. Akiva Eiger would be lenient if we know that there are two hairs.