Scott Kelly was an extensively decorated Navy pilot who retired from active duty at 48. That makes sense, right? Carrier landings; being a test pilot... that's a young man's game. Except that after he retired from a distinguished career as a fighter pilot, he became... an astronaut.
Kelly turned 52 in orbit. His Russian cosmonaut partner, Mikhail Kornienko is even older; he turned 55 early in their history-making mission.
Fun fact: Kelly was briefly two-inches taller than his twin identical twin brother Mark Kelly (also an astronaut). That was the result of his spine decompressing in space, but the effect is short lived. Speaking of short lived, because the International Space Station is traveling to fast in orbit, Kelly is technically about 1/100th of a second younger than he would be, had he spent the past year on earth.
Comparing Scott and Mark will help NASA learn about the effects of prolonged space travel on the human body. But 50-something space travelers like Scott and Mikhail are not nearly the oldest-ever astronauts. That record is held by John Glenn, who was a mission specialist on the Space Shuttle in 1998, at the age of 76.