Mark Spitz
Before Michael Phelps, American swimmer Mark Spitz held the record for most Olympic Gold Medals in a single year with seven in 1972. He set world-record times in all seven events and likely could have continued his success four years later, but he instead chose to retire at the tender age of 22. He never swam competitively again, with the exception of a 1992 stunt in which he once again tried to qualify for the Olympics at age 41.