Mishnah Niddah 5:8: What are her signs [which indicate that she has matured]? Rabbi Yose the Galilean says: once the fold appears beneath the breast. Rabbi Akiva says: once the breasts drop. Ben Azzai says: once the areola darkens. Rabbi Yose says: [when the breasts develop] such that if one puts one's hand on the nipple it sinks and gradually returns.

אֵיזֶהוּ סִימָנֶיהָ. רַבִּי יוֹסֵי הַגְּלִילִי אוֹמֵר, מִשֶּׁיַּעֲלֶה הַקֶּמֶט תַּחַת הַדָּד. רַבִּי עֲקִיבָא אוֹמֵר, מִשֶּׁיַּטּוּ הַדַּדִּים. בֶּן עַזַּאי אוֹמֵר, מִשֶּׁתַּשְׁחִיר הַפִּטֹּמֶת. רַבִּי יוֹסֵי אוֹמֵר, כְּדֵי שֶׁיְּהֵא נוֹתֵן יָדוֹ עַל הָעֹקֶץ וְהוּא שׁוֹקֵעַ וְשׁוֹהֶה לַחֲזֹר:

Mishnah Niddah 5:9: A woman of twenty years who did not produce two [pubic] hairs, should bring evidence that she is twenty years old, and she is an aiylonit [a woman with arrested sexual development who cannot bear children]: she does not take part in chalitzah [the ceremony performed as an alternative way to release the widow of childless man from the obligation to wait for Levirate marriage], nor yibum. A man of twenty years who did not produce two [pubic] hairs, should bring evidence that he is twenty years old, and he is a eunuch: he does not take part in chalitzah, nor yibum, according to Beit Hillel. Beit Shammai say: this and that one both [are considered to be infertile if they have not produced hairs] at the age of eighteen. Rabbi Eliezer says: regarding the male it is in accordance with Beit Hillel, and regarding the female it is in accordance with Beit Shammai, since a woman arrives [at maturity] earlier than a man.

בַּת עֶשְׂרִים שָׁנָה שֶׁלֹּא הֵבִיאָה שְׁתֵּי שְׂעָרוֹת, תָּבִיא רְאָיָה שֶׁהִיא בַת עֶשְׂרִים שָׁנָה, וְהִיא אַיְלוֹנִית, לֹא חוֹלֶצֶת וְלֹא מִתְיַבֶּמֶת. בֶּן עֶשְׂרִים שָׁנָה שֶׁלֹּא הֵבִיא שְׁתֵּי שְׂעָרוֹת, יָבִיא רְאָיָה שֶׁהוּא בֶן עֶשְׂרִים שָׁנָה, וְהוּא סָרִיס, לֹא חוֹלֵץ וְלֹא מְיַבֵּם, אֵלּוּ דִּבְרֵי בֵית הִלֵּל. בֵּית שַׁמַּאי אוֹמְרִים, זֶה וָזֶה בְּנֵי שְׁמֹנֶה עֶשְׂרֵה. רַבִּי אֱלִיעֶזֶר אוֹמֵר, הַזָּכָר כְּדִבְרֵי בֵית הִלֵּל, וְהַנְּקֵבָה כְּדִבְרֵי בֵית שַׁמַּאי, שֶׁהָאִשָּׁה מְמַהֶרֶת לָבֹא לִפְנֵי הָאִישׁ: