There is a halakhic puzzler. Halakhah requires married women to cover their hair and it is considered a biblical requirement. BT Berakoth 24a: "R. Shesheth said: A woman's hair is a sexual incitement, as it says, Thy hair is as a flock of goats." אמר רב ששת שער באשה ערוה שנא' (שיר השירים ד) שערך כעדר העזים
There is near unanymity that this refers to a married woman. Thus, married woman have to cover their hair, unmarried women--whether 13 33 or 99--don't (never been married, really). I say near unanymity, because there've been communities which required unmarried women to cover their hair as well.
This leads to some puzzlement. If female hair is ערוה, sexual incitement how can it possibly be that this is dependent upon marriage? Clearly this is not a typical sort of ערוה; it obviously isn't like the breasts. So what gives?
The best answers are all apologetic. They amount to some variation that seeing the hair is supposed to be "private for the husband" or perhaps that allowing unmarried women to go bare headed is a concession of sorts (to snare a husband?).
But suppose one takes a positive-historical approach--paranthetically, the 19th century European interpretation of Judaism (or movement) called Positive-Historical Judaism, a term coined by R. Zecharias Frankel, has nothing to do with a positive outlook (as Wikipedia, R. Berel Wein and others understand it (quote to be supplied later). It is true that "positive-historical" sounds like "we like this Judaism thing, but we're going to be historical about it," but it doesn't mean "we like this Judaism thing." It refers to Positivism, a 19th century theory which promotes the belief that only knowledge gained through the application of scientific method is true (or proved true). Okay, I know little about Positivism and hope I didn't botch that up. But that is indeed what Frankel meant. He meant that the kind of Judaism he promoted was to employ the scientific method in the study of its sacred texts, laws and customs. Not "I like Judaism; I feel positive toward it," which of course he did.
In any case, taking a positive-historical approach one might wish to know what באשה in שער באשה ערוה meant at the time it was said by R. Shesheth. In other words, is an "isha" only a married woman? Whether the answer is yes or no, one also would wish to know when were women getting married in R. Shesheth's time? Could it be that then, before the age of delayed adulthood, girls got married at about the onset of adolescence? If so, the passage is explained fairly well. There is no problem of how a 20 year old woman's hair is not ערוה so long as she is not yet married, but once she marries then--overnight--that same hair becomes ערוה. Rather, R. Shesheth is basically saying that much like obviously sexual parts of a woman, hair is also ערוה. By "woman" he may well have meant "married woman," but that might have simply been exactly the same thing as a sexually developing young girl. Which is to say, hair covering is to begin with puberty, more or less.
I know that it will be objected on historical grounds that the Talmud very precisely defines puberty in relation to physical development. If so, then how could "the onset of adolescence" or "the time when girls get married" be meant? Well, I need to think about that. :) In any case, it seems to me that using a positive-historical approach (assuming that the historical evidence bears out my tentative assumption that the time girls got married was at the time of physical development) the halakhic requirement actually called for no discrepancy between women of all ages who didn't marry and those who did. Hair would not "become" ערוה with marriage so much as with physical maturation. Realize also that such young marriage was certainly practiced in medieval Ashkenaz, and even as late as the 19th century in parts of the Jewish world.
It is not unfortunate, I suppose, that if this was the "original intent" that it became forgotten in time until the gap between the onset of when women actually begin covering their hair and when girls actually begin to become women widened. This only meant that now that we delay adulthood (and marriage) our young ladies are not burdened with a burdensome practice that even many adult women find challenging.
(Of course this whole pilpul can probably be undone from a positive-historical perspective by noting that "minhag Lita" was not to cover the hair for about a hundred years. ;)