Why is (action-adventure) anime so focused on teenagers? I've been a fan of anime since Star Blazers in my childhood, but... I wonder about an overused cliche in anime action-adventure series: teenage prodigy heroes.

For example, the most popular action-based anime (as opposed to comedy or relationship-based) currently on Netflix and Hulu (as well as StackExchange tag frequency): Attack on Titan, Black Butler, Bleach, Death Note, Fairy Tail, Fullmetal Alchemist, Knights of Sidonia, Naruto, One Piece, Sword Art Online. All of them feature unusually capable teens as the lead protagonists. Adults are supporting characters, or entirely absent.

Certainly there are exceptions (Samurai Champloo, etc), and this trope is also seen in the US (e.g. Adventure Time, Avatar, Young Justice, etc) but it's a matter of degree. Non-child heroes are not a rarity in western animation (Avengers, Batman, Tranformers, etc).

Is this a real thing in Japanese culture, and if so what does it mean? Or is it just a USA-based artifact of marketing (i.e. this particular subgenre is actively emphasized for translation) or audience (i.e. non-child anime is just as common but has less active fans)?
Action/Adventure anime is typically catagorized under shounen - which means adolescent males.
People like other people who are like themselves. In this case, adolescent males empathize and therefore like shows about other adolescent males being heroes.

The "exceptions" you listed are typically listed under seinen, which don't target just young people.

As for the difference, there are plenty of novels written for young adults featuring teen heroes, just look at Hunger Games or the slew of vampire books or harry potter. You just see so much more of it in Japan because there are so much more teenagers reading manga, reading novels, and watching anime.

Please see these questions for more information - What are the differences between types of anime? or Why is most anime centered on fighting?