Late 19th Century: Age <20
At the end of the 19th century, the body of a young girl was pulled from the Seine. She appeared healthy and there were no marks of violence on her, so it was assumed she had drowned herself. The custom in those days was to put unidentified bodies on display at the Paris morgue, and the public were invited to come by to view them in the hope that somebody might recognized one of the corpses. Nobody did. But a morgue worker was impressed enough by her beauty that he made a plaster cast of her face.
Weirdly enough, word of her beauty spread. Many copies of the cast were produced, and it caught the public imagination. Although her identity was never discovered, Camus, Rilke, and Nabokov were three of at least a dozen writers who wrote her or her mask into their novels, plays, and short stories. It became the "thing" to have a cast of her face, presumably to hang on the wall, stare at, and sigh romantically about the cruelty of life and the mysteriousness of her smile. According to one critic, "a whole generation of German girls modeled their looks on her." Another says, "The Inconnue became the erotic ideal of the period, as Bardot was for the 1950s."
If the face looks familiar, perhaps you have taken a First Aid course. A commonly-used CPR teaching dummy, "Rescue Annie" (also called "CPR Annie" and "Resusci Anne"), was developed in 1958 using her face as the model.
Sources: Wikipedia, The Girl from the Seine