Many reports say a 21-year-old Blue Origin passenger is now the youngest woman to have flown to space. An 18-year-old woman rode a Virgin Galactic rocketplane last year, but didn’t go quite as high.

There is no single definition of "astronaut," leading to debate over who qualifies for the title. Reaching the Kármán line (62 miles above sea level) is a common benchmark, but NASA and the Air Force ...