Why does the Mishnah not include the Chalitzah of a Ketanah along with the
others in the list of Chalitzos that are invalid? Moreover, once the Mishnah
teaches that the Chalitzah of a Ketanah is invalid, why does it have to add
the words "she must do Chalitzah when she grows up?" It is obvious that she
must do Chalitzah again when she grows up, because her Chalitzah done as a
Ketanah is invalid!
(a) TOSFOS (105b) explains that the reason why the Chalitzah of a Katan is
invalid is because, as the Gemara says, the Torah uses the term "Ish"
(Devarim 25:7) with regard to Chalitzah. The Torah does not mention the word
"Ishah" with regard to Chalitzah, and therefore we might have thought that a
woman does not have to be an adult in order to do Chalitzah. However, a
Hekesh is made between an Ish and an Ishah, and thus the Ishah must also be
an adult in order to do Chalitzah.
Tosfos says that we might have thought that this comparison is only
mid'Rabanan, since the Torah does not explicitly compare the two, Ish and
Ishah. Because of this, when the Mishnah says that a Ketanah's Chalitzah is
invalid we might have thought that it is only invalid mid'Rabanan, but not
mid'Oraisa. The Mishnah goes out of its way to add the extra words "she must
do Chalitzah when she grows up" in order to show that the Chalitzah of a
Ketanah 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 most of the Rishonim say
that the Girsa in the Mishnah should be, "if she did not do a Chalitzah
[later when she grows up], her Chalitzah [that she did when she was a
Ketanah] is *Kesheirah*" -- and not "Pesulah!" According to this Girsa,
indeed the comparison between Ish and Ishah which teaches that a Ketanah may
not do Chalitzah is only a comparison mid'Rabanan; mid'Oraisa, though, the
Ketanah's Chalitzah *does* work. Therefore, if b'Di'eved she does not do
another Chalitzah when she grows up, the Chalitzah she did as a Ketanah is
effective. That is why the Mishnah lists the Chalitzah of a Ketanah separate
from the other types of Chalitzos Pesulos.
(c) The TIFERES YISRAEL suggests that even according to our Girsa, which says
that "if she did not do a Chalitzah [later when she grows up], her Chalitzah
[that she did when she was a Ketanah] is *Pesulah*," the Mishnah might be
telling us that the Chalitzah of a Ketanah is Pesulah only mid'Rabanan and
not mid'Oraisa.
He bases this on the Gemara at the beginning of Sukah that says that when
discussing the act of a Mitzvah d'Oraisa which is not valid, the Mishnah
records it as "Pasul." In contrast, when discussing the act of a Mitzvah
d'Rabanan which is not valid, the Mishnah does not say that it is "Pasul,"
but rather the Mishnah gives the way to rectify the invalid act.
Here, the Mishnah says that the Ketanah should do Chalitzah when she grows
up. The Mishnah is stating the way to rectify the invalid Chalitzah that she
did as a Ketanah. This shows that the Mishnah holds that the Chalitzah of a
Ketanah is invalid only mid'Rabanan (for if it was invalid mid'Oraisa, the
Mishnah would have said simply that it is Pesulah without giving the way to
rectify it).
(We may add that Tosfos might have come to the opposite conclusion -- that
the Mishnah is implying that the Chalitzah of a Ketanah is invalid mid'Oraisa
-- because he is following his own reasoning elsewhere. In the beginning of
Sukah, Tosfos explains that the Mishnah's style of recording the way to
rectify a Mitzvah when discussing a Mitzvah d'Rabanan applies only where the
Mishnah says *only* the way to rectify it, without adding that if it is not
rectified the act is "Pasul.")