The description used for a girl who is under the legal age for sex, usually reserved for those under 14 but developing.
From the statement that having sex with a underage girl can land you with a sentence of 25 years to life in jail, depending on country.
Johnny : "look there, shes hot"
Peter : "shes Twenty five years!"
Johnny : "but shes hot"
Peter: "shes too young to have my wicked way with!"