Edgar Jimenez Lugo was arrested in Cuernavaca in December. Investigators say he confessed to working for the Beltran Leyva brothers' cartel and to killing four people whose beheaded bodies were hung from a bridge.
Lugo says he started working for the cartels when he was 11. He allegedly posted brazen photos online.
A Mexican judge Tuesday sentenced Lugo to three years in prison for organized crime, homicide, kidnapping, as well as drug and weapons charges. Lugo received the maximum sentence allowed for a minor in the state.