Babes Together

Girl has meant any young unmarried woman since about 1530. Its first noted meaning for sweetheart is 1648. The earliest known appearance of girl-friend is in 1892 and girl next door, meant as a teenaged female or young woman with a kind of wholesome appeal, dates only to 1961.
• Usage for adults
The word girl is sometimes used to refer to an adult female. This usage may be considered derogatory or disrespectful in professional or other formal contexts, just as the term boy can be considered disparaging when applied to an adult man. Hence, this usage is often deprecative. It can also be used deprecatively when used to discriminate against children ("you're just a girl").
In casual context, the word has positive uses, as evidenced by its use in titles of popular music. It has been used playfully for people acting in an energetic fashion (Canadian singer Nelly Furtado's "Promiscuous Girl") or as a way of unifying women of all ages on the basis of their once having been girls (American country singer Martina McBride's "This One's for the Girls"). These positive uses mean gender rather than age.