Head Covering Source, Talmud Bavli, Nedarim 30b

Translation significantly done by Sarah Mulhern, some additions by me.  
Text:

משחורי הראש - אסור בקרחין ובעלי שיבות, ומותר בנשים ובקטנים, שאין נקראין שחורי הראש אלא אנשים

גמ'. מ"ט? מדלא קאמר מבעלי שער.

ומותר בנשים ובקטנים, שאין נקראין שחורי הראש אלא אנשים. מ"ט? אנשים זימנין דמיכסו רישייהו וזימנין דמגלו רישייהו, אבל נשים לעולם מיכסו, וקטנים לעולם מיגלו

Translation:

Mishnah: He who vows not to benefit from black-haired people may not benefit from bald people and gray haired people, but may benefit from women and children, because only men are called black haired.

Gemara: What is the reason for this ruling?  Since it did not say "from those who have hair" [therefore the bald- who might once have had black hair- are still forbidden].
"But may benefit from women and children" because only men are called black-haired.  What is the reason?  Men sometimes cover their heads and sometimes reveal their heads, but women's hair is always covered, and children are always bareheaded.

Commentary:

This short text is a font of information about norms around head-covering in [at least some part of] the Talmudic period.  The Gemara text, which is where things get interesting for us, begins as an inquiry into the reason behind a somewhat surprising ruling in the Mishnah- that if one vows not to gain benefit from black-haired people, one may not get benefit from men who are either black-haired or might once have been so (which makes hair color an identity that one retains even without hair...  fascinating).

The Gemara continues to elucidate that the reason that someone who has made this vow may still derive benefit from women and children is that women's hair is always covered, while children are always bare-headed- but neither gets called "black-haired".  This makes more sense in relation to women than to children, but oh well...  However, the sense in regard to women is that you can't be expected to identify them by hair color, since you will never see their hair or know its color.  Interestingly, it says that All women have covered hair- seemingly even unmarried ones.  (Admittedly, it seems highly unlikely that there were many never-married women at all.)  It is only men who may or may not cover their head- both women and children have social expectations regarding their head/hair.

This is very much a description of a social reality- there are Jewish communities (like our own contemporary community) where some women are bare-headed, and others (such as the Yemenite Jewish community, historically) where children of both genders also had covered heads.  But the assumption that All women cover their hair, while only some men do, is an interesting perspective, in comparison with what we now presume.