jailbait

noun

1. (slang, uncountable, usually)A sexually mature person below, or appearing to be below, the legal age of consent, who is regarded, usually by an adult, as being sexually attractive and/or seductive.

   “"The first time she modelled for Edward Weston, in March 1934, Charis Wilson knew she didn’t look good. At 20 she was “a piece of jailbait”, a mere child, especially with the stumpy plaits into which she sometimes twisted her hair."”