Can anyone think of

a 20th century American coming of age story with a female protagonist?

I never thought about it. But my friend Molly pointed out that McCullers's The Heart is a Lonely Hunter is one of the only ones (and even then, the female is just one of a a few main characters and the others are male). Mick is a pre-teen awkward kind of tomboyish girl who smokes cigarettes and climbs things and has a severe physical and emotional reaction when she listens to classical music. She doesn't even have a girl's name really. Just something to think about.