In 1878 Jules Verne, author of immensely popular YA novels, published a story about a 15 year old captain:
"Dick Sand is a fifteen year old boy serving on the schooner "Pilgrim" as a sailor; he is left in charge of the ship with no experienced sailors to help him."
Fast forward 132 years, and you'll find that YA characters in traditionally adult professions are doing just fine:
"A 15-year-old boy turned himself in to Peruvian police and confessed to killing 10 people, apparently making him Peru’s youngest hitman, local media reported Thursday.
Known by the nickname “Gringacho” and not further identified by authorities, the presumed hitman said that he was recruited by a criminal network in the city of Trujillo, 570 kilometers (353 miles) north of Lima, when he was just 12 years old.
From what he told the police, over the past three years he killed a total of 10 people, among then a young woman who was eight months pregnant." SOURCE