John Glenn was 77 years old when STS-95 launched in 1998.
Glenn was born on July 18, 1921, and STS-95 lifted off on October 29, 1998. He was therefore 77 at launch, having celebrated his 77th birthday a little over three months earlier. The mission made him the oldest person to reach Earth orbit at the time.
His age was not merely a publicity detail. NASA and medical researchers were interested in comparing Glenn’s response to spaceflight with data from his earlier Mercury mission and with younger astronauts. The crew conducted experiments involving sleep, balance, cardiovascular function, immune response, and other effects of microgravity and aging.
A frequent mistake is to confuse Glenn’s age at launch with his age during his first orbital flight. In 1962, he was 40 when Friendship 7 carried him around Earth. Another distinction matters today: later people became older than Glenn during suborbital flights, but Glenn remains the oldest person to reach Earth orbit. STS-95 thus connected the early U.S. space program with the mature Shuttle era.