The details surrounding his death are still unclear but drug overdose is suspected as officials found "numerous empty pill bottles" in the area where Sage's body was discovered, TMZ reported quoting sources.
The police said they found Sage in the home while responding to a 'welfare check', however his lawyer George Braunstein said he was found by a housekeeper.
Paramedics were called immediately but they pronounced him dead at the scene. The authorities are reportedly trying to determine if the death was intentional or accidental.
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.