Reference for Nedarim 3:8

הַנּוֹדֵר מִשְּׁחוֹרֵי הָרֹאשׁ, אָסוּר בַּקֵּרְחִין וּבְבַעֲלֵי שֵׂיבוֹת, וּמֻתָּר בַּנָּשִׁים וּבַקְּטַנִּים, שֶׁאֵין נִקְרָאִין שְׁחוֹרֵי הָרֹאשׁ אֶלָּא אֲנָשִׁים:

If one bevows himself from "the black of head," he is forbidden to the bald and the grey, [not having spoken of those who have hair], and he is permitted to women and children, for only men are called "the black of head." [For men sometimes conceal their heads and sometimes reveal and "blacken" them, whereby it is seen that they are men. But women always go with their heads covered, and (young) children, both male and female, go with heads uncovered and cannot be (readily) distinguished as male or female. For this reason, only grown men are called "the black of head."]

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