Mishnah Niddah 6:11: A female child who produced two [pubic] hairs may participate in either in chalitzah or yibum, and she is obligated in all of the commandments stated in the Torah. And similarly, a male child who produced two [pubic] hairs is obligated in all of the commandments stated in the Torah. And he is fit to become a wayward and rebellious son from the time he produces two hairs until his beard encircles; [this refers to] the lower [beard] and not the upper one, but the Sages spoke in a euphemism. A female child who has produced two hairs cannot do mi'un [the refusal of a marriage by a fatherless child married off by her mother or brother, retroactively annulling the marriage]. Rabbi Yehuda says: [she may still do mi'un] until the black [i.e. the hair] increases.

תִּינוֹקֶת שֶׁהֵבִיאָה שְׁתֵּי שְׂעָרוֹת, אוֹ חוֹלֶצֶת אוֹ מִתְיַבֶּמֶת, וְחַיֶּבֶת בְּכָל מִצְוֹת הָאֲמוּרוֹת בַּתּוֹרָה. וְכֵן תִּינוֹק שֶׁהֵבִיא שְׁתֵּי שְׂעָרוֹת, חַיָּב בְּכָל מִצְוֹת הָאֲמוּרוֹת בַּתּוֹרָה. וְרָאוּי לִהְיוֹת בֵּן סוֹרֵר וּמוֹרֶה, מִשֶּׁיָּבִיא שְׁתֵּי שְׂעָרוֹת עַד שֶׁיַּקִּיף זָקָן, הַתַּחְתּוֹן וְלֹא הָעֶלְיוֹן, אֶלָּא שֶׁדִּבְּרוּ חֲכָמִים בְּלָשׁוֹן נְקִיָּה. תִּינוֹקֶת שֶׁהֵבִיאָה שְׁתֵּי שְׂעָרוֹת, אֵינָהּ יְכוֹלָה לְמָאֵן. רַבִּי יְהוּדָה אוֹמֵר, עַד שֶׁיִּרְבֶּה הַשָּׁחֹר:

Mishnah Niddah 6:12: The two hairs spoken of regarding a red heifer [i.e. that two white or black hairs invalidate it, see, Parah 2:5], and [those spoken of] regarding nega'im [diseased patches on skin, clothes, or houses that create impurity; i.e. that two hairs on a blemish are a sign of impurity, see Nega'im 4:4], and regarding those spoken of anywhere, [the hairs must be long] enough to bent their tips to [touch] their roots, according to Rabbi Yishmael. Rabbi Eliezer says: [long] enough to be pinched by a finger-nail. Rabbi Akiva says: [long] enough to be removed with scissors.

שְׁתֵּי שְׂעָרוֹת הָאֲמוּרוֹת בַּפָּרָה וּבַנְּגָעִים, וְהָאֲמוּרוֹת בְּכָל מָקוֹם, כְּדֵי לָכֹף רֹאשָׁן לְעִקָּרָן, דִּבְרֵי רַבִּי יִשְׁמָעֵאל. רַבִּי אֶלְעָזָר אוֹמֵר, כְּדֵי לִקְרֹץ בַּצִּפֹּרֶן. רַבִּי עֲקִיבָא אוֹמֵר, כְּדֵי שֶׁיְּהוּ נִטָּלוֹת בְּזוּג: