The younger Stallone's attorney, George Braunstein, confirmed his death to the New York Post, but did not disclose the cause.
The newspaper quoted sources it identified only as being close to the case as saying a prescription drug overdose is suspected, but it was unclear whether it was accidental or a suicide.
"I have just gotten a report that he was found dead at his home," Braunstein told the Post of Sylvester Stallone's child with his ex-wife Sasha Czack.
"He was in good spirits, and working on all kinds of projects," said Braunstein. "He was planning on getting married. I am just devastated. He was an extremely wonderful, loving guy. This is a tragedy."
A Los Angeles Police Department representative told the Post there was "no suspicion of foul play or criminal activity."
Police discovered Stallone's body after someone asked them to check on him because he hadn't been in touch.
Sage Stallone made his big-screen debut alongside his father in "Rocky V."
Sylvester Stallone's publicist, Michelle Bega, told TMZ the action star is "devastated and grief-stricken over the sudden loss of his son, Sage Stallone."
"His compassion and thoughts are with Sage's mother, Sasha," Bega said. "Sage was a very talented and wonderful young man. His loss will be felt forever."