The phrase "underage girl" commonly refers to a female who is younger than the legal age of consent or maturity determined by authorities in a particular country or region. Full definition
Within six months of our marriage he started having sex with underage girls. (premierchristianity.com)
Sexualised images of underage girls in beauty pageants are presented in the same publications, accompanied by sub-headings that project a sexual «knowingness» onto their behaviour. (politics.co.uk)
Actress Emily Browning's outfit is particularly egregious and almost borderline pedophiliac, much like a lot of those anime drawings of underage girls in scanty outfits that are so disturbing at so many levels for Westerners. (scifimoviepage.com)