"Girl" isn't a pejorative unless you're determined to take it that way. It has nothing to do with age; it's part of an informal nomenclature, as in "girls and guys". I know many women who refer to themselves as "girls" -- including those in their fifties and sixties -- and/or use "grrl" in their online names, and there are even groups like LinuxChix which use "chicks" in a hip sense rather than a belittling one. "Girls' night out" is another example. I would even observe there's an inflection point where women above a certain age prefer the term and consider it a compliment to be referred to as such, as youth and vitality are desirable connotations.

And groups of women do refer to wanting to find more "boys" -- the terms "men" and "women" both seem to connote some kind of stodginess, probably because most people's models for what men and women are, growing up, is parents, teachers, and other authority figures, all of whom are tragically unhip.