The website quoted law enforcement sources as saying that the death did not appear to be a suicide as there was no suicide note found at the scene.
Stallone is "devastated and grief-stricken" by the death of his eldest son.
"Sylvester Stallone is devastated and grief-stricken over the sudden loss of his son Sage Stallone. His compassion and thoughts are with Sage's mother, Sasha. Sage was a very talented and wonderful young man. His loss will be felt forever," Stallone's representative Michelle Bega said.
Braunstein told the New York Post that Sage's death came as a shock. "He was in good spirits, and working on all kinds of projects. He was planning on getting married. I am just devastated. He was an extremely wonderful, loving guy. This is a tragedy...'I think it was probably some sort of accident," he said of the death.
Sage co-starred with his father in two films. He was the first of two sons Stallone had with first wife Sasha Czack. Sage made his acting debut in 1990's Rocky V as Stallone's onscreen son and also appeared with his father in 1996's Daylight.