Mishnayos Niddah Perek 5 Mishnah 9

A girl twelve years and one day old who grew two pubic hairs is classified as a young woman. Six months later, she becomes a grown woman. But a woman who is twenty years old who did not grow two pubic hairs and was never classified as a young woman shall bring proof that she is twenty years old, and from that point forward she assumes the status of a sexually underdeveloped woman [ailonit], who is incapable of bearing children. If she married and her husband died childless, she neither performs ḥalitza nor does she enter into levirate marriage, as the mitzva of levirate marriage applies only to a woman capable of conceiving a child. An ailonit is excluded from that mitzva. In the case of a man who is twenty years old who did not grow two pubic hairs, they shall bring proof that he is twenty years old and he assumes the status of a sexually underdeveloped man [saris], who is excluded from the mitzva of levirate marriage. Therefore, if his married brother dies childless, he neither performs ḥalitza nor enters into levirate marriage with his yevama. This is the statement of Beit Hillel. Beit Shammai say: For both this case of a woman and that case of a man, they shall bring proof that they are eighteen years old, and they assume the status of a sexually underdeveloped woman and man respectively. Rabbi Eliezer says: The status of the male is determined in accordance with the statement of Beit Hillel, i.e., he assumes the status of a sexually underdeveloped man at the age of twenty; and the status of the female is determined in accordance with the statement of Beit Shammai, i.e., she assumes the status of a sexually underdeveloped woman at the age of eighteen. The reason is that the woman is quick to reach physical maturity, and reaches that stage before the man reaches physical maturity.

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: