Father Sylvester Stallone and mother Sasha Czack reunited to lead the procession at St. Martin of Tours Catholic Church in Brentwood, Los Angeles.
Their son was found dead on July 14 at his Hollywood apartment. The results of an autopsy have been deferred pending toxicology tests.
Czack requested that mourners make a donation to Autism Speaks instead of bringing flowers, E! News reports. Sage's younger brother Seargeoh is autistic.
Close friend Stephen Wozniak has said that he refuses to believe Sage could have taken his own life, adding: "He was a wonderful guy who loved life and was so sensitive that it got to him."
Czack has suggested recent dental surgery could have led to his death, while Sylvester has hired a private investigator to investigate the circumstances.