Hair Covering & Single Women
Although in our day only married women are required to cover their hair, there was once a custom for even single women to do so, as well. In fact, no less an authority than the Rambam rules that single women are required to cover their hair in public even today. There is also an obscure opinion that according to kabbala a single woman should always cover her hair. Nevertheless, most authorities reject these opinions and interpret them to be referring to women who were once married but are now "single," i.e. divorcees and widows. Indeed, it is virtually unanimous that women who were never married are not required to cover their hair. While a woman covering her hair is essentially an indication as to whether or not she is a virgin, it is unclear if a woman who was raped is required to cover her hair, as well.
In many Chassidic communities there are strict dress codes which include how unmarried girls are to wear their hair. This is based on the opinion that although unmarried girls are not required to cover their hair, they must nevertheless wear their hair in a modest and unassuming manner, such as in a ponytail or by braiding it. According to this approach, young women are not to style their hair in accordance with the latest trends and fashions. Other authorities rule that single girls are not subject to any restrictions on how they choose to wear their hair. Although common custom is in accordance with this view, it goes without saying that a woman must always dress and conduct herself with a sense of modesty.
 Rambam, Hilchot Issurei Biah 21:17.
 Levush, EH 21:2.
 There are some authorities who permit divorcees and widows to cease covering their hair in specific circumstances. See: Igrot Moshe, EH 1:57; Machazeh Eliyahu 118-120
 Chelkat Mechokek, EH 21:2; Be'er Heitev, EH 21:5.
 Ketubot 15.
 Be'er Heitev, EH 21:5; Pitchei Teshuva, EH 21:2.
 Magen Avraham 75:3.
 Shulchan Aruch Harav, OC 75:4.
 Even Yisrael 7:8; Kaneh Bosem 1:6.