Don Grady died of cancer June 27 in Thousand Oaks, California at the age of 68.. His survivors included Virginia Lewsader, his wife of 27 years and two children, Joey and Tessa.
His first television appearance according to imdb.com was in the Spin and Marty series in 1957 for Walt Disney.
He appeared in many television westerns, before appearing on My Three Sons (1960 -1971) at the age of 16 and was 27 when the show ended in 1971.
Grady became involved with music after the run of My Three Sons ended, His last acting appearance was in an episode of Simon and Simon in 1964.
He has acted only four times in the last 41 years, since he ended his run as being Robbie Douglas.
The following obituary from the Los Angeles Times tells more details of his life.