A bopper, in the sense you meet in older informal English, is another word for a teenybopper: a very young teenager, often a girl, who is crazy about pop stars, pop songs, and the look that goes with them. It belongs to talk from about the 1960s and 70s and sounds dated now; people under thirty almost never say it. The picture is screaming fans, posters on the wall, and music that grown-ups thought was silly. Though actually, jazz people have used bopper in a different way, for someone into bebop; that is not this sense. Here it is the pop-fan meaning, usually a bit teasing, not cruel if said lightly, but it can sound as if you are not taking the young person seriously.