Sage Stallone, Son of Sylvester Stallone, Found Dead at 36 Kevork Djansezian/AP Photo Sage Stallone, son of actor Sylvester Stallone, was found dead in his apartment today, ABC News Los Angeles station KABC reported. He was 36. The cause of his death was not clear. I'm devastated. He's an extremely bright, creative person," Sage Stallone's longtime attorney, George Braunstein, told KABC. Sage Stallone played Rocky's son, Robert Balboa, in "Rocky 5."