The oldest astronaut is John Glenn Jr. (USA), who was 77 years 103 days old when he was launched into space as part of the crew of Discovery STS-95 on 29 October 1998. The mission lasted 11 days, landing on 7 November 1998.
Senator Glenn was one of the first US astronauts to fly with the Mercury orbiter space programme. In February 1962 he completed a five hour, three-orbit flight aboard Friendship 7
Comments below may relate to previous holders of this record.