Why is (action-adventure) anime so focused on teenagers?
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?