Words, like the humans who use them to communicate, evolve with time. This is simply because the meanings we attach to words change over time as the circumstances that gave them birth change. Once, the word “girl” was unisex and used to describe either a young male or female. Then, in the 15th century, English did what it does best in word acquisition and pilfered the term “boy,” which the French had been using as a pejorative for “slave” or “one of low birth,” and began using it to refer to young males exclusively.
This is a companion discussion topic for the original entry at http://spectrummagazine.org/node/11895