Mishnah Niddah 1:4: Who [is considered] a virgin [such that her hour suffices]? Any who has never seen [menstrual] blood all her days, even if she is married. A pregnant woman? [She is considered to be pregnant] once her fetus becomes known [i.e. at the end of her first trimester]. A nursing woman? [She is considered to be nursing] until she weans her child [i.e. for twenty-four months]. If she gave her child to a nursemaid, or weaned him, or if he died [within twenty-four months], Rabbi Meir says: she is rendered impure [retroactively] from that time to that time [on the previous day, i.e. twenty-four hours earlier]. And the Sages say: her hour [of discovering blood] suffices [to render her impure only from that moment].

אֵיזוֹ הִיא בְתוּלָה, כֹּל שֶׁלֹּא רָאֲתָה דָם מִיָּמֶיהָ, אַף עַל פִּי שֶׁנְּשׂוּאָה. מְעֻבֶּרֶת, מִשֶּׁיִּוָּדַע עֻבָּרָהּ. מֵנִיקָה, עַד שֶׁתִּגְמֹל אֶת בְּנָהּ. נָתְנָה בְנָהּ לְמֵנִיקָה, גְּמָלַתּוּ אוֹ מֵת, רַבִּי מֵאִיר אוֹמֵר, מְטַמְּאָה מֵעֵת לְעֵת. וַחֲכָמִים אוֹמְרִים, דַּיָּהּ שְׁעָתָהּ:

Mishnah Niddah 1:5: Who is [considered] an elderly woman? Anyone who has gone through three seasons [without menstruating], approaching her old age. Rabbi Eliezer says: any woman [i.e. young or old] who has gone through three seasons [without menstruating], her hour [of discovering blood] suffices [to reckon her impurity from that moment]. Rabbi Yose says: a pregnant or nursing woman who has gone through three seasons [without menstruating], their hour suffices [to render them impure only from the moment of discovering blood].

אֵיזוֹהִי זְקֵנָה. כֹּל שֶׁעָבְרוּ עָלֶיהָ שָׁלשׁ עוֹנוֹת סָמוּךְ לְזִקְנָתָהּ. רַבִּי אֱלִיעֶזֶר אוֹמֵר, כָּל אִשָּׁה שֶׁעָבְרוּ עָלֶיהָ שָׁלשׁ עוֹנוֹת, דַּיָּהּ שְׁעָתָהּ. רַבִּי יוֹסֵי אוֹמֵר, מְעֻבֶּרֶת וּמֵנִיקָה שֶׁעָבְרוּ עֲלֵיהֶן שָׁלשׁ עוֹנוֹת, דַּיָּן שְׁעָתָן: