The age used to refer to girls that are under the age of 18, no matter how old they look, to refer to the jail time one would get from having sex with them.
dude 1: OH MAN! check out that hot chick over there.
dude 2: no way man, shes 5-10
dude 1: What? she has to be at least 16...
dude 2: no man, 5-10, trust me.