While NASA doesn't have any limits on how old its astronauts need to be to visit space, it does require potential candidates to have a bachelor's degree in a STEM-related field such as biology, engineering or computer science. NASA also requires that all astronaut trainees be U.S. citizens with excellent vision, though the use of glasses is acceptable. And finally, successful candidates must have either 1,000 hours' worth of piloting experience in a jet aircraft or three years of relevant work experience [source: NASA].
But no, there are no official age restrictions. In the past, NASA has selected candidates who were as young as 26 and as old as 46 [source: NASA]. And the late astronaut John Glenn returned to space in 1998 at the age of 77.
Now, the odds of actually getting chosen, however, are ... well, they're astronomical. In 2016, NASA announced that a handful of new people would be allowed to enter its astronaut training program. The administration was then flooded with more than 18,300 applications. Only 12 of these hopefuls were chosen [source: Ward]. That's an acceptance rate of just 0.065 percent!
Normally, the selection process consists of two rounds. NASA begins by interviewing about 120 of the most promising candidates. From that pool, the top eight to 14 applicants enter the newest official class of astronaut candidates [source: Ziv].
Basic training for NASA astronaut candidates takes up to two years. Those who graduate become full astronauts, but that doesn't mean they head right into space (keep reading). So far, there have been 22 classes of NASA astronauts, many of which received cutesy nicknames. For example, the classes of 1996 and 1998 were called the "sardines" and "penguins," respectively [source: Wattles].