Can be a number of things. Mostly it's the new flavor of the week, they are called this til they graduate to "Baby Momma" or "The Wife." Often times, old girl is a called this because she is kept on the DL and/or your ashamed of her.
Example: "I think old girl put something in my drink"
A “14 year old girl” will seek attention by any means possible, like fake depression and nudes. Not all 14 year old girls are like this but most of them are. Be smart, don’t act like a 14 year old girl.
Example: Person: yo I heard Bethturned14 today
Person 2: she’s gonna be a total 14 year old girl
A girl who thinks she is old enough to do whatever she wants, sometimes going through the "scene" phases. She tries to fit in with older people usually. Most of the time this person looks older than 13 or tries to act older than 13, she usually dates people older than her. She tries to beadventurous and tries new and dangerous things.
Example: David: "dude! that girl is sooo hot"
Friend: "she's a 13 year old girl.."
David: "omg, she looks so much older than that"