The Italian-born author, polo enthusiast and owner of several hair salons died Monday following a battle with prostate cancer, the newspaper said. He published his first book, "Stewart Lane," last May.
"He was in the best 90-year-old-man shape," Scott Swerdlin, president of Palm Beach Equine Clinic in Wellington, told the Post. "He was a very kind gentleman. A lot of fun. He enjoyed his wife, enjoyed his family. He loved to watch polo."
The newspaper said it was unable to reach members of Stallone's family for comment regarding his death.
He is survived by his wife, Kathleen, and children, Sylvester, Frank Jr., Carla, Bryan and Dante.
Stallone is divorced from Sylvester's mother, astrologer Jackie Stallone.
Celebrity Families of 2014 [PHOTOS]