Eager Young Space Cadet

For those of you who came here looking for the term's idiomatic meaning, that's a Cloudcuckoolander.

As for actual space cadets:

A Discredited Trope from the early Pulp Magazine days of media science fiction (peaked in the 1950's in movie serials and TV shows), involving a hero who was part of an organization that handles law and order in outer space, much like in a Western... in space. Frequently the titular Space Cadet was a child or teen who was a new recruit or a sidekick to an older hero. May be the equivalent of Walking the Earth with isolated outposts and frontier planets; especially strange when the Kid Hero seems not to have a family at home.


 * Space Cadet: Robert A. Heinlein novel, best known for its opening pages being so prophetic as to not read like Science Fiction today.
 * His earlier story Misfit also fits the trope, and makes the inspiration from the Depression-era Civilian Conservation Corps more explicit.
 * Tom Corbett, Space Cadet (based on Heinlein's novel)
 * Space Patrol (1950's radio and TV)
 * Captain Video and his Video Rangers
 * Rocky Jones, Space Ranger
 * The Lucky Starr series of books by Isaac Asimov, which started with David Starr, Space Ranger.
 * Jeff Wells of Isaac and Janet Asimov's Norby series is also technically a space cadet, although most of his adventures are unofficial ones.
 * Rod Brown of the Rocket Rangers
 * Buck Rogers, especially in the comic strip and movie serial, where the kid sidekick Buddy was added.
 * Commando Cody.
 * Buzz Lightyear from Toy Story is an homage/parody of this trope.
 * "Galaxy High" plays this trope semi-straight.
 * Porky Pig's title in the Looney Tunes short Duck Dodgers in the 24th and 1/2 Century (and the series based on it) is actually Eager Young Space Cadet.
 * Various midshipmen throughout David Weber's Honor Harrington stories, including Harrington herself in the novella "Ms. Midshipwoman Harrington".
 * Wesley Crusher represents the best example in the Star Trek universe. As a child prodigy, he represented an Audience Surrogate for a young viewer and could be considered a "sidekick" to Picard. Later in the series, Wesley became a literal cadet in Starfleet. Harry Kim is the closest example in Star Trek: Voyager, while "Red Squad" in Deep Space Nine is something of a Deconstruction of the trope.
 * Sasami was cast as this in the Tenchi Muyo! OVA special Mihoshi's Space Police Adventure. Of course, that's not all she was there for...