Gemara

104b (Mishnah): If a Ketanah did Chalitzah, she does Chalitzah again when she becomes an adult. If not, the Chalitzah was invalid.

105b (Rav): This is like R. Meir. He learns from "Ish" that the Yavam must be an adult, and he equates her law to his. Chachamim agree that he must be an adult, but she may be an adult or minor.

Chachamim hold like R. Yosi (who does not learn similar matters from each other);

(R. Chiya or R. Shimon bar Rebbi): One who prays must look down (towards the Shechinah in Eretz Yisrael) - "My eyes and heart will always be there";

(The other of R. Chiya and R. Shimon): His eyes should face up - "We will lift our hearts to our hands".

R. Yishmael bar R. Yosi citing R. Yosi: His eyes should be down and his heart up, to fulfill both verses.

A Yevamah came in front of Rebbi. He told Avdan to check whether or not she is an adult.

R. Yishmael bar R. Yosi: My father said that the Torah requires the Yavam to be an adult, but she may be an adult or minor.

Rebbi: We need not check her. The elder has already ruled!

(R. Ami): From R. Yosi, we learn that a Yevamah may do Chalitzah if she reached the age of Pa'utos (six to 10, depending on how sharp she is).

(Rava): She must have reached the age of vows (this will be explained).

The Halachah is, she may not do Chalitzah until she brings two hairs (of adulthood).

Gitin 65a (Rava): When a boy reaches the age of vows, his vows are binding. if he made his property Hekdesh, it takes effect. A girl of the corresponding age may do Chalitzah.

Nidah 45b (Mishnah): If a girl above 11 (full) years vows, we check her (to see if she understands that Nedarim are to Hash-m);

If a girl above 12 vows, it is valid (we do not check her).

Rishonim

Rif and Rosh (12:13): The Mishnah says that if she did not do Chalitzah again after maturity, her Chalitzah was void. (Rosh - the Rif's text says 'Kosher', like the Yerushalmi.) Rav taught that this is like R. Meir, but Chachamim allow Chalitzah of a Ketanah. Rava says that she must have reached the age of vows. The Halachah is, she must bring two hairs.

Nimukei Yosef (DH Gemara): Our Mishnah says that if she did not do Chalitzah again after maturity, her Chalitzah was Kosher. This is like R. Meir, unlike R. Eliezer. A Tosefta says that if she did not do Chalitzah again after maturity, R. Eliezer disqualifies and Chachamim are Machshir.

Chidushei Anshei Shem (2): Rav taught that R. Meir equates her to him! We must say that this is mid'Rabanan.

(Nimukei Yosef, ibid): However, the Halachah is that she must bring two hairs. Perhaps we disqualify even b'Di'eved, like R. Eliezer. This is not clear, so one must be stringent.

Question (R. Tam, in Tosfos 105b DH Rava): Rava himself taught 'the Halachah is, she must bring two hairs.' In Gitin, Rava taught that a girl at the age of vows may do Chalitzah!

Answer #1 (R. Tam): There, Rava taught according to R. Yosi.

Answer #2 (Ri, in Tosfos, ibid.): The text in Gitin should say 'Rabah'.

Answer #3 (Nimukei Yosef DH Rava): There are two concepts of 'the age of vows'. During the 12th year, we check her. After 12 years, her vows are binding (without checking her). Since it says here 'until she brings two hairs', this implies that 'the age of vows' refers to before (the time for hairs, i.e.) 12 years. In Gitin, when Rava says 'the age of vows' he refers to after 12 years.

Rambam (Hilchos Yibum 4:16): If a Ketanah did Chalitzah to an adult, it is Pasul.

Magid Mishneh: Some texts say 'Kosher'. Clearly, the correct text is 'Pasul'.

Lechem Mishneh: The Halachah is, she must bring two hairs. This is unlike Chachamim, who do not equate her to the Yavam. Rather, it is like R. Meir. However, we do not hold like R. Meir who says that Chalitzah of a Katan is Pasul, rather like Chachamim, who say that it is totally void. The Tur says so in the name of the Rosh. However, the Rambam says that Chalitzah of a Ketanah is Pasul. He explains that the Halachah follows R. Yosi. Mid'Oraisa, Chalitzah of a Ketanah is Kosher. Mid'Rabanan, we require two hairs. The Magid Mishneh alludes to some who say that b'Di'eved it is Kosher. The Rambam (1:16,17) said that a boy cannot do Chalitzah until he matures, and likewise a girl. It seems that he holds like R. Meir, who equates them mid'Rabanan.

Poskim

Shulchan Aruch (EH 169:43): If a Ketanah did Chalitzah, it is Pasul. It is unlike Chalitzah of a Katan, which is totally void and does not forbid Yibum.

Shulchan Aruch (49): If a Ketanah did Chalitzah to an adult, it is Pasul. It disqualifies her from (doing Yibum with) the brothers.

Tur: The Rosh equates her to him (therefore Chalitzah of a Ketanah is totally void).

Question (Beis Yosef DH u'Mah she'Chosav Rabeinu): Tosfos (105b DH Ketanah) says so. However, we do not find that the Rosh says so. And even Tosfos was unsure, for the Yerushalmi says that if she did not repeat Chalitzah after maturing it was Kosher! The Rif's text also says 'Kosher', but it seems that since the Bavli requires two hairs, before this is Pasul.

Rebuttal (Bach DH va'Adoni and Drishah 85): Tosfos explains R. Meir, who says that Chalitzah of a Katan or Ketanah is Pasul! Tosfos says that it is 'nothing', i.e. even mid'Oraisa it does not permit her l'Shuk, but mid'Rabanan it forbids Yibum.

Defense (of Beis Yosef - Beis Shmuel 49): Tosfos rules like Chachamim who say that Chalitzah of a Katan is totally void, and like R. Meir, who equates her to him.

Answer #1 (Korban Nesan'el (on Rosh) 70): The text of the Rosh should say 'also the Rif's text says so (Pasul, i.e. mid'Rabanan. Mid'Oraisa, the Chalitzah is void, like that of a Katan). The Yerushalmi's text is 'Kosher'.' However, the simple understanding of the Rosh is like the Rif.

Answer #2 (Drishah 85 DH u'Leyashev): The final words of the Rosh, 'the Halachah is, she must bring two hairs' shows that we do not distinguish a Ketanah from a Katan like Chachamim do. Rather, she is like a Katan, and Chachamim say that his Chalitzah is void.

Rema: When she matures she must do Chalitzah with one of them. She does not need Chizur (to repeat Chalitzah with every brother).

Terumas ha'Deshen (225, cited by Gra 136): Even though some require Chizur after a Pasul Chalitzah, we may be lenient here. According to Tosfos, the first Chalitzah was totally void. According to the Yerushalmi, the first Chalitzah was valid. It is a mere stringency to do Chalitzah after maturity, so we are not stringent to do Chizur. Even though the Rambam holds that the first Chalitzah was Pasul, we may join to Tosfos and those who have the Yerushalmi's text (the Rosh says that the Rif does) the opinion that never requires Chizur. We may rely on them, since Chizur is totally mid'Rabanan.