Gemara

(Beraisa): A case occurred in which Reuven sold his father's property, and Reuven died. They (members of the family) said that the sale was invalid, for he was a minor at the time. They asked if they may check the body.

R. Akiva: Firstly, you may not check the body, for it is a disgrace. Secondly, signs often change after death.

156a (Rav Nachman): We check boys above 13 (and girls above 12) for hairs regarding Kidushin, divorce, Chalitzah, and Mi'un (retroactive annulment of a marriage mid'Rabanan).

Nidah 46a (Rava): A girl may do Mi'un anytime until 12 full years. After 12, she may not do Mi'un or Chalitzah.

Question: He forbids Mi'un because he assumes that she is an adult (and the marriage became mid'Oraisa). If she is an adult, she should be permitted to do Chalitzah!

Rava is not in Safek whether she is an adult, for he taught that we need not check a girl above 12. She is Muchzekes to be an adult.

Answer #1: Normally, we assume that she is an adult. She may not do Chalitzah in a case when we see that she does not have hairs now. She may not do Mi'un, lest she brought hairs after 12 years, and they fell out.

Answer #2: We did not check her. Regarding Chalitzah we are concerned (lest she is still a minor). Rava's Chazakah refers to Mi'un (we are stringent not to allow Mi'un). She may not do Chalitzah until we see hairs.

(Rav Dimi of Neharde'a): The Halachah is, we are concerned lest they fell out (regarding Mi'un) only if she had relations with her husband after 12 years of age, for then it is a Safek mid'Oraisa). If not, she may do Mi'un (since she is married only mid'Rabanan).

(Beraisa - R. Yehudah): Women do not check her during the Perek (her 12th year).

During the Perek she is not believed (that she found hairs), for Rava's Chazakah does not apply.

(Beraisa - R. Shimon): Women check her during the Perek.

He holds that hairs during the Perek do not make her an adult. If these hairs are still there after the Perek, we will not use them to establish her to be an adult.

(Beraisa): A woman is believed to be stringent, but not to be lenient.

Some say that R. Yehudah taught this. He refers to checking during the Perek. Some say that R. Shimon taught it, about checking after the Perek.

Rishonim

Rambam (Hilchos Edus 9:8): If a 13 year old has Simanim above, we need not check him. If not, we do not accept his testimony until he is checked.

Rambam (Hilchos Ishus 2:15): If a boy above 13 did not bring a Siman below, and he brought all the Simanim above, it is a Safek whether he is a minor of adult. If he was not checked below, since he has Simanei Bagrus above, he is assumed to be an adult.

Rosh (33:5): Once a child reached the years of Bar Mitzvah, we establish that he was an adult from the required number of years. Even though we conclude that Chazakah d'Rava is only for Mi'un, i.e. to be stringent, when one was checked later and was found to be an adult, we establish her to be an adult from the age of 13. A proof of this is from Bava Basra 155. A youth sold and died, and the buyers wanted to check if he had hairs. R. Akiva ruled that it would not help, for we are concerned lest they changed after death. If he were alive, checking now would establish that he was an adult from the time of the sale. Another proof is from 156b. We check for Kidushin, divorce, Chalitzah, and Mi'un. If a girl accepted Kidushin, and later we find that she has hairs, if she accepted Kidushin afterwards from another man, we ignore it. If we do not find hairs, we are concerned for both acts of Kidushin (perhaps she had hairs, and they fell out). R. Yitzchak says that we checked her before the second Kidushin, for if we checked her afterwards, even if we find hairs she needs a Get from both, for perhaps the hairs came after the second Kidushin, like Shmuel says in Kidushin 79. However, there we discuss Simanim of Bagrus (on the day a girl becomes a Bogeres), and there is no Chazakah that they came in the morning. When we find hairs of Na'arus, we say that they were there from 12 years. R. Yitzchak says so even according to Shmuel, and all the more so according to Rav, who assumes that Simanei Bagrus came in the morning. The Halachah follows Rav.

Poskim

Shulchan Aruch (EH 155:19): A girl may do Mi'un after 12 years if we checked her and she did not bring two hairs. Stam, after 12 years she may not do Mi'un, for there is a Chazakah that she brought two hairs. In any case she may not do Chalitzah until we check her and find two hairs.

Gra (66): Even though according to one answer R. Shimon argues with Rava, the Halachah does not follow R. Shimon. The rule is, we follow R. Yehudah against R. Shimon.

Shulchan Aruch (CM 35:1): If a man has a full beard we need not check whether or not he has two hairs.

Source (Beis Yosef DH v'Im): Tosfos (Yevamos 80b d v'Hevu says so). The Tur brings this in EH 169. Here he calls it 'sprouting of the beard', which means a full beard. Why didn't he say a full beard, like he said there? Perhaps only there, for Chalitzah, we require a full beard. However, what is the source to distinguish? The Rambam (Hilchos Edus 9:8) says that if a 13 year old has Simanim above, we need not check him. However, he holds that the beard is a Siman.

Rebuttal (Gra 5): Tosfos connotes unlike this! Rather, the Tur learned from the Rambam (Hilchos Ishus 2:15).

Shulchan Aruch (OC 460:1): One may not knead Matzas Mitzvah through a Nochri, Cheresh, lunatic or minor.

Taz (1 DH v'Cheresh): Here, minor refers to one who lacks Da'as (understanding). The Rashba (1:26, cited in Beis Yosef d Ein) says that he must understand when they tell him to make it Lishmah. The Maharil requires Bar Mitzvah; this is a big stringency.

Eliyahu Rabah (2): 'Minor' means before 13 years.

Chak Yakov (3): 'Minor' means before 13 years. Also the Maharil says so. The Taz disagrees, but the Poskim equate this to Gitin, Chalitzah and Shechitah, which require adults. I have seen people use minors to make holes in the dough with adult supervision. This is better, for many helpers speeds the process. One should be careful (to use adults) if possible.

Mishbetzos Zahav (1): Chak Yakov connotes that we require 13 years and two hairs. The Bach connotes that guarding l'Shem Mitzvah is mid'Rabanan. The Pri Chodosh connotes that it is mid'Oraisa. It is not clear what the Rambam (Hilchos Chametz u'Matzah 5:3,8) holds. Therefore, one should not assume that they brought two hairs right after 13 years, rather, until they are older. Matzah for Yom Tov Sheni is just like for the first night.

Bi'ur Halachah (DH Ein): L'Chatchilah one should not rely on Chazakah d'Rava for Matzas Mitzvah.

Bi'ur Halachah (11:2 DH Tavan): Since the Torah requires spinning the threads for Tzitzis Lishmah, it is the same as guarding Matzah for the Mitzvah. We should not let a boy or girl past 13 or 12 do it, unless we know that he or she brought two hairs. B'Di'eved one may rely on Chazakah d'Rava.

Sha'arei Teshuvah (39:1): The Noda b'Yehudah (2:1) says that if a boy just over 13 years wrote Tefilin, and they did not check him for hairs and he is not in front of us, the Tefilin are Kesherim. We rely on Chazakah d'Rava b'Di'eved, but not l'Chatchilah.

Bi'ur Halachah (DH Oh): In Choshen Mishpat we say that if a man's beard filled out, we know that he brought two hairs. The Magen Avraham says that after 18 we need not be concerned lest he did not bring hairs; all the Acharonim rejected this.