1) The Rama at the end of the Hil Purim writes that there is no problem of lo tilbash gever simlas isha for a boy to dress up like a girl or vica versa because “ain m’chavnin elah l’simcha b’alma,” it’s just done in fun and jest. What kind of excuse is that to allow an issur d’oraysa?
This Rama seems to be proof to position of the Bach (see Shach sk 7 and Taz #4 in Y”D 188) that the issur of lo tilbash is violated only where the intent is to mimic the opposite gender. Since in this case the clothing is being worn only for Purim fun, that intent is lacking and therefore there is no issur. The same idea might apply to clothing worn purely as a functional necessity, e.g. snowpants for skiing, sweatpants for exercise, a doctor wearing scrubs, etc.
2) The gemara (Nidah 46) quotes in the name of Rava that once a girl has turned 12 we assume she has reached the age of maturity. The gemara then qualifies Rava’s din: this is true only for mi’un (i.e. once she is 12 she can no longer do mi’un) but not for chalitzah (i.e. we do not assume at 12 that she is an adult and can do chalitzah).
Why should there be a difference between mi’un and chalitzah? True, mi’un is a din derabbaban and chalitzah is a din d’oraysa, but Rava is telling us that there is a chazakah, and we rely on chazakos when it comes to dinim d’oraysa. Why not here?
There are three theories to explain the difference, with major nafka minos l’halacha. The first two attempt to justify distinguishing between dinim d'oraysa and derabbanan; the third takes a different approach entirely:
A) When speaking about reaching physical maturity, there are a significant number of people who are exceptions to the norm, a miyut ha’matzuy, and therefore we do not assume Rava’s chazakah is strong enough proof with respect to dinim d’oraysa.
B) A chazakah should be relied upon only where facts cannot be discovered, but where it is efshar l’varer, one must check. Since in this case checking would be intrusive and embarrassing, when it comes to dinim derabbanan the consideration of kavod habriyos overrides the need to do an examination.
C) The Noda b’Yehudah (II:1) and R’ Akiva Eiger (#13) both argue that the distinction here has nothing to do with d’oraysa vs. derabbanan. The difference between mi’un and chalitzah is this: in the case of mi’un, chazakah d’Rava, which precludes mi’un, reinforces the status quo chazakah of the girl being married. In the case of chalitzah, our baseline starting point is that the girl is too young to do chalitzah and therefore has a chezkas issur yevamah la'shuk. Chazah d’rava challenges the status quo; it means the girl can undo her existing status. The gemara is telling us that chazakah d’Rava carries weight when it has another chazakah/status quo on its side; when it runs counter to another chazakah, it does have the strength to overturn it.
Can a 13 year old boy write his own pair of tefillin? If theory #1 or #2 is right (assuming we cannot or do not want to do an examination), then the tefillin would be pasul. Since we are dealing with a din d’oraysa, chazaka d’Rava is not strong enough proof. However, if theory #3 is correct, there is no chezkas issur in this case that we need to counteract, and therefore, chazakah d’Rava is proof enough that the 13 year old has reached maturity. (See R’ Akiva Eiger who rejects this argument for a different reason).
Can a 13 year old boy be motzi the tzibur in parshas zachor? Same issue. Based on theory #1 and theory #2, he is disqualified. Based on theory #3, his leining would be OK.
3) It seems like every other post I write lately needs some correcting -- sorry about that. I wrote two weeks ago that the Aruch haShulchan seems in one place to opine that a wife can be yotzei mishloach manos with her husband's gift, yet in another place he says women are independently chayavos to give their own gifts. I think in all likelihood in the later statement the Ah"S is just echoing the Rama aliba d'dina; in the former statement he is speaking practically, echoing the MG"A who comments on that Rama that the minhag is for women to be included in their husband's gift. When people give mishloach manos my impression is that most people write something like "From the Ploni family" on their gift; they don't designate a gift as being from Mr. Ploni and one from Mrs. Ploni. This would follow the view of the MG"A.