Niddah, Chapter 6, Mishnah 12. The closing mishnayos of this chapter deal with the physical signs by which the Torah recognizes that a child has become an adult. Our mishnah returns to that subject and lists what changes in a person's halachic status once those signs appear.
The mishnah begins with the girl: "Tinokes shehevi'ah shtei se'aros", a young girl who has produced two hairs. Once she has reached the age of twelve and two hairs have appeared in the genital area, she is a full adult in every respect.
One practical result is stated first: she is fit for either chalitzah or yibum. This refers to a woman whose husband died childless, so that she now stands before her brother-in-law and falls into the category of one who must either be taken in marriage by him (yibum) or released through the ceremony of chalitzah. A girl who has reached this stage of maturity is competent for either path; the choice is open to her, because she is now legally an adult.
Then comes a wider statement: "vechayeves bechol mitzvos ha'amuros baTorah", every command recorded in the Torah now binds her. Her new standing is not confined to questions of marriage and release from it; from this point onward the entire body of mitzvos rests upon her shoulders.
The same is true of the boy: "vechein tinok shehevi shtei se'aros", a boy who has reached thirteen and has produced two hairs. He too is "chayav bechol mitzvos ha'amuros baTorah", obligated in every mitzvah written in the Torah.
For the boy the mishnah records an additional outcome: "vera'ui lihyos ben sorer umoreh", he now qualifies for the status of the rebellious son that the Torah presents in Parshas Ki Seitzei. This status, however, lasts only for a limited window: "mishehevi shtei se'aros ad sheyakif hazakan", beginning with the appearance of two hairs and ending when the "beard" has come around, and the mishnah pins down which beard is meant: "hatachton velo ha'elyon", the lower and not the upper. Facial hair is not the marker at all; what is being described is hair in the genital region, once it has spread so as to encircle that area. Why call it a beard? The mishnah supplies the answer: "ela shedibru chachamim bilashon nekiyah", the Sages preferred a delicate turn of phrase and spoke in refined language.
The mishnah then turns to a further consequence of the girl's maturity: "tinokes shehevi'ah shtei se'aros einah yecholah lema'ein", a girl who has produced two hairs can no longer perform mi'un.
Mi'un is a concept that belongs to a rabbinic form of marriage. A young girl could be married off in her childhood by her father, or, after his death, by her brother, the purpose being to protect her and to keep her out of the hands of unscrupulous men. That arrangement, however, came with a condition: as the girl approached maturity she retained the right to protest the marriage and thereby be released from it. Our mishnah teaches that this right has a deadline. Once the physical signs of adulthood have appeared, she can no longer object. Her refusal had to have been voiced while she was still a ketanah, a minor girl, and not once she had already become bas mitzvah.
Rabbi Yehudah disagrees about where that line falls. He holds that although the two hairs make her an adult, the laws of mi'un continue to apply somewhat longer: "ad sheyirbeh hashachor", until the black hair has grown abundant in that area. Not two hairs, then, but a fuller growth, which indicates that she is older. Until that point she may still exercise mi'un; from then on she may not.
The mishnah thus draws two hairs into the center of a person's legal life: they open the door to obligation in all the mitzvos, to chalitzah and yibum, to the category of ben sorer umoreh, and they close the door on a young girl's power of refusal.