Samuel Clemens (a.k.a Mark Twain), age 15, 1850. Daguerreotype taken when he was a printer in Hannibal, Missouri.
Check out that baller belt-buckle! Are you as cool as fifteen-year-old Sam?
Update: That’s not a buckle! From a reader on Facebook: “He’s holding, with his right hand on its pistol grip (remember, these images are reversed), a typesetter’s composing stick, in which the metal type for his name are inserted, ready to print. He was smart enough to know that the image would be reversed, and thus show correctly in the photo!”