Longman Dictionary of Contemporary English
Wiktionary
n. 1 (context chiefly British English) An alumna, especially of a public school. 2 (context UK English) Affectionate term of address for a woman.
Usage examples of "old girl".
Mark just wanted him to think he was having sex with a gorgeous nineteen-year-old girl.
Hall was mean for giving homework on Fridays, all the ways Debra Gilhooly had of being such a bitch, how she and Pepsi had planned to go trick-or-treating as Spice Girls last Halloween and Mom had said Pepsis Mom could do whatever she wanted, but no nine-year-old girl of hers was going out trick-or-treating in a short skirt, high heels, and a cammi top.
Sometimes you just had to be an O'Neal, even if you were a thirteen-year-old girl.
Not because of some eleven-year-old girl I've never even laid eyes on.
Come, old girl, let bygones be bygones, and gie us a kiss, and we'll go to bed.