A long time ago, we were all GIRLS...

In the prologue to the Canterbury Tales in the late 1300s, Geoffrey Chaucer speaks of the "young girls of the diocese," not merely young women.

Back in the Middle English period, when the word "girl" first arose in the language, it was used to signify "child," regardless of the gender of the kid in issue. 

That didn't alter until the early 15th century, when the word "boy," which was assumed to have been acquired from French roughly a century earlier as another name for a slave or a man of low birth, came to be used more broadly for any young male. As boy encroached on its meaning, girl was compelled to modify or face extinction.