Does it encompass any specific age group? (young, middle-aged, elderly, all of them)
I heard it in an old film and read it once in sentences like:
"Come on, old girl, cheer up."
"Who is that old girl you were talking to?"
old girl – a familiar name used to refer to a woman
EDIT – So as to avoid multiple interpretations of the idiom, and too many answers, let's keep this question to its use in BrE.
Best Answer
There are essentially two distinct and different uses.
Former pupils of schools are sometimes known as 'Old Boys' and 'Old Girls'. David Cameron, the present Prime Minister is an Old Boy of Eton College. Princess Anne is an Old Girl of Benenden School in Kent. The use may be becoming dated, with more and more schools converting to co-ed, and the increasing use of 'former pupil'. (How boring!)
Both Old Boy and Old Girl, are hearty backslapping terms of endearment among social equals, more often in the upper strata of society. It is a bit like the working-class expression 'mate', but in a higher social register. Old Boy, especially, has a military officer ring about it. Again one senses that calling people old girl or old boy has been in decline for several decades.
Though it is non-pejorative I would counsel any visitor to Britain to take care how they use it. It is a term which could sound slightly preposterous if used in an inappropriate register.