Kid Hero

Western Animation

 * Adventure Time has Finn, Flame Princess, and several other borderline examples. Finn, the main protagonist and card-carrying hero, is a somewhat more realistic take on this trope, as he is genuinely nice and has good intentions but often screws up out of ignorance or impulsiveness. He doesn't always do the right thing, either, although he does when it counts. Even though Finn ages during the series (he begins at 12 and celebrates his 17th birthday in the final seasons) and grows up quite a bit, he never quite abandons his boyishness.
 * Twins Dipper and Mabel from Gravity Falls are 12, celebrating their 13th birthday in the series finale. Once Dipper finds the journal describing the oddities of Gravity Falls, he and Mabel get involved with increasingly weird phenomenon.
 * Steven Universe stars Steven, a prepubescent boy (starts the series at age 12, ends it at age 14) who protects the Earth with his mother's team of Magical Girl Warriors, all of whom are ageless alien gemstones.
 * The sequel series (Future) deconstructs the trope, revealing that everything Steven experienced has given him PTSD, and that saving the universe two years prior has left him an anxious wreck who's struggling to understand what to do with his life now that no one needs him to be the hero.
 * Connie, who's canonically a year and a quarter younger than Steven, later becomes a full-fledged member of said team of Magical Girl Warriors after taking sword lessons from Pearl.
 * Teenaged Lars also becomes one after he Took a Level in Badass in season 5. To wit, he goes from a cowardly Jerkass to a Guile Hero and Space Pirate, as well as the Arch-Enemy of a high-ranking Homeworld Gem.
 * Sadie also has a couple of Badass Normal moments, the most major being when she took down a giant, invisible corrupted Gem by herself, while already injured and armed with nothing but a crude wooden fish spear.