Humans are physiologically well adapted to life on Earth. Consequently, manned space flight has many negative effects on the body.
However, that has not been an impediment for astronauts with ages that would be associated with retirement in many countries.
On October 29, 1998, John Glenn was 77 years and 103 days old when he embarked on a space mission, aboard the Discovery STS-95. After nine days, on November 7, 1998, Glenn returned to Earth.
He was also the first American astronaut who circled the Earth, aboard the Frienship 7 ship in 1962.
Peggy Whitson became the oldest astronaut with 56 years and 282 days. On November 18, 2016, he left for the International Space Station (ISS) aboard a Soyuz MS-03, which took off from the Baikonur cosmodrome and entered orbit after eight minutes.
Interestingly, Whitson is also the woman who has done the most spacewalks. And on March 30, 2017, it equaled the record of more time accumulated in space walks: 50 hours and 40 minutes, previously set by his NASA partner Sunita Williams.