This is a rude, informal word. People — often adults talking in a joking or ugly way — use it for someone who is still under the legal age for sex. The picture inside the word is cruel: that person is “bait,” and if an adult goes after them, the adult may end up in jail. Though actually, that picture puts the blame in the wrong place. The young person is not a trap. The adult is the one who must follow the law. The age itself is sloppy too: it is not the same in every country (16 in some places, 18 in others), and the fine rules can be complicated. You will hear the word in films, songs, and careless talk. Decent speakers do not use it about a real person, because it treats a child or teenager as a thing that tempts you.