Why do girls only start covering their hair when they get married and not earlier like boys? I have heard sources from the Rambam and other Sephardic Rishonim that seem to say that girls should cover their hair. I can’t understand why a girl making a Bracha with hair uncovered can be seen as being Yirat Shamayim.

Shalom UVerachah, Women are required to cover their hair as hair is considered to be "Ervah" meaning a part of the body that should be covered. Men have the custom to cover their heads out of a sense of yirat shamayim in light of the story in the Gemara (Shabbat) that covering one's head increases the fear of Hashem. There were certain places where women covered their hair even as girls but this is not the custom today, and hair covering is required after marriage, as in the Mishnah in Ketubot. Kol Tuv