Geoffrey Chaucer wrote about 'girls of the diocese' - but at that time, 'girl' meant any young person - what today we might call 'youngsters'. Then, in the early fifteenth century, 'girl' began to apply only to females, and the new word, 'boy' had to be invented for male children. 'Boy' is thought to have come from a French word meaning 'slave' or 'peasant'.
Maybe there is another shift in usage occurring now, where the upper age limit for a 'girl' is gradually increasing.
