Question:

Why must a married woman cover her hair according to Judaism? I have heard many things said including that it is because the hair is very holy and therefore must be covered? Is this true? And if so why not cover it already from birth?

Answer:

I am not aware of any source which states that the hair of a woman is considered holy and I believe this is getting confused with the following Kabbalistic explanation which does explain why a woman must cover her hair and why specifically it is to be covered and not completely shaved. From it we can also deduce why it must only be covered starting from after marriage:

The Kabbalistic explanation: A married woman is considered an Erva and she is hence required to cover her hair, in order to prevent the hair from giving nurture to the unclean forces. Women come from the level of severities, similar to Levites, and hence their hair represents a surplus of severities similar to that of the Levites. Hence, just as the Levites had to shave their hair to prevent the unhealthy nurture of evil that can derive from the surplus, so too, a married woman must cover her hair to prevent this. Now, as for why a married woman is not required to shave her hair similar to the Levites, this is because a certain limited level of severities are necessary for this world to function, and hence her hair is not to be shaved but merely covered. As for why specifically a married woman, as opposed to a girl who was never yet married, is required to cover her hair, seemingly, this is because the aspect of severities only becomes expressed in a woman after the union of marriage.

Sources:  Likkutei Torah Ki Seitzei p. 36b; See also Likkutei Torah Beshalach 1a; 24a; Emor 32a; Nitzavim 52a; Derech Mitzvosecha Mitzvas Tiglachas Metzora 104b; Likkutei Sichos 18:447; Igros Kodesh 15:415; See Sefer Shaar Hamitzvos of Arizal on Parshas Kedoshim “however a woman comes from the level of Za… And all of the grasping of the severities and their root is in her, and if you remove the severities completely from the head of the female then they will be completely nullified, and therefore a woman is not to shave her head and on the contrary she’s to grow out her hair as is well known, to leave a root for the severities”; Ben Yehoyada Miseches Brachos p. 61 and Miseches Yuma p. 47; Shemen Sasson 9