10 August 1965
Karen Muir became the youngest official world record-holder in swimming history when she recorded 1min 8.7sec for the 110 yards backstroke at the Derby Baths, Blackpool, England. At the age of twelve, she became not only the youngest world record-holder in swimming, but she is up to now the youngest athlete ever to set a world record in any sports code. Muir set fifteen world records and was voted into the International Swimming Hall of Fame, Fort Lauderdale, in 1980. Because of the Olympic Games ban on South Africa, she never competed at the Olympics and retired in 1968.
References

Wallis, F. (2000). Nuusdagboek: feite en fratse oor 1000 jaar, Kaapstad: Human & Rousseau.