1) THE CHALITZAH OF A KETANAH
QUESTION: The Mishnah lists different types of invalid Chalitzos, such as the Chalitzah performed by a Cheresh (a deaf Yavam), a Chareshes (a deaf Yevamah), and a Katan (minor). The Mishnah adds that if a Ketanah performs Chalitzah, her Chalitzah is invalid and she must perform Chalitzah again when she reaches adulthood.
Why does the Mishnah not include a Ketanah's Chalitzah in the list of invalid Chalitzos? Moreover, once the Mishnah teaches that a Ketanah's Chalitzah is invalid, why does it need to add that "she must perform Chalitzah when she reaches adulthood"? Since the Chalitzah she performs as a Ketanah is invalid, it is obvious that she must perform Chalitzah again when she reaches adulthood.
(a) TOSFOS (105b) explains that the reason why the Chalitzah of a Katan is invalid is, as the Gemara explains, that the Torah uses the term "Ish" (Devarim 25:7) with regard to Chalitzah. Since the Torah does not mention the word "Ishah" with regard to Chalitzah, one might have thought that a woman does not need to be an adult in order to do Chalitzah. However, a Hekesh is made between an "Ish" and an "Ishah," and thus the woman also must be an adult in order to do Chalitzah.
Tosfos points out that one might have thought that the comparison of "Ish" and "Ishah" is only mid'Rabanan, since the Torah itself does not explicitly compare the two. Accordingly, when the Mishnah states that a Ketanah's Chalitzah is invalid, one might have thought that it is invalid only mid'Rabanan, but mid'Oraisa it is valid. The Mishnah therefore adds the extra words, "she must perform Chalitzah when she reaches adulthood," in order to show that a Ketanah's Chalitzah is completely invalid, even mid'Oraisa.
(b) TOSFOS (ibid.) in his second answer, the RAMBAM (Perush ha'Mishnnayos) cited by the RASHBA, and the RASHBA in the name of the Rishonim explain that the proper text of the Mishnah should read, "If she does not perform Chalitzah [later when she reaches adulthood], her Chalitzah [performed when she was a Ketanah] is Kesherah" -- and not "Pesulah." According to this Girsa, the comparison between "Ish" and "Ishah" -- which teaches that a Ketanah's Chalitzah is invalid -- is a comparison only mid'Rabanan. Mid'Oraisa, the Ketanah's Chalitzah is valid. Therefore, if, b'Di'eved, she does not perform another Chalitzah when she reaches adulthood, the Chalitzah she performed as a Ketanah is effective. This is the intention of the Mishnah when it lists the Chalitzah of a Ketanah separate from the other types of invalid Chalitzos.
(c) The TIFERES YISRAEL suggests that even according to the Girsa (as it appears in our text of the Mishnah) that reads, "If she does not perform Chalitzah [later when she reaches adulthood], her Chalitzah [performed when she was a Ketanah] is Pesulah," the Mishnah may mean that the Chalitzah of a Ketanah is Pesulah only mid'Rabanan and not mid'Oraisa.
He bases this assertion on the Gemara in the beginning of Sukah (2a) which teaches that when the Mishnah discusses an invalid attempt to fulfill a Mitzvah d'Oraisa, it calls the act "Pasul." In contrast, when the Mishnah discusses an invalid attempt to fulfill a Mitzvah d'Rabanan, the Mishnah does not call the act "Pasul," but rather it describes how to rectify the invalid act.
The Mishnah here states that the Ketanah should perform Chalitzah again when she reaches adulthood -- that is, it teaches how to rectify the invalid act of Chalitzah which she did as a Ketanah. The Mishnah's description of how to rectify the invalid act indicates that the Mishnah maintains that the Chalitzah of a Ketanah is invalid only mid'Rabanan. (If the Mishnah would maintain that the act is invalid mid'Oraisa, it would say simply that the act is "Pesulah" and not describe how to rectify it.)
(Apparently, the reason why Tosfos reaches the opposite conclusion -- that the Mishnah implies that the Chalitzah of a Ketanah is invalid mid'Oraisa -- is that he follows his own reasoning there in Sukah. Tosfos in Sukah explains that the Mishnah's style of describing how to rectify an invalid act of a Mitzvah d'Rabanan applies only where the Mishnah mentions only the way to rectify it and does not add that the act is "Pasul" if it is not rectified.)