You might count the April 1958 "National Invitational Tournament" as a world championship. It was a triple round robin with four players: Mosconi, Jimmy Moore, Irving Crane and Luther Lassiter. Mosconi and Moore tied at 10-2 and Mosconi won the playoff. Mosconi was 44. (info from Charlie Ursitti's files)
Mosconi was talked into playing in the 1966 "World Tournament" in Burbank. It was an 18-player round-robin, if you can imagine. He finished second to Balsis. In that tournament Balsis set a new BPI average record of 22. Mosconi would have been 52 at that time.
The last "officially" titled world championship Mosconi won was in April 1956 in Kinston, NC. It was an 8-player double round robin and he was undefeated. Crane was second with a 10-4 record. The order of finish was: Mosconi, Crane, Moore, Lassiter, Fitzpatrick, Eufemia, Rudolph, Riggie. Mosconi was 42 at the time.