Kiddie Kid

Anime & Manga

 * Lampshaped in Cute High Earth Defense Club LOVE! with Yumoto. He is 15, but generally acts much younger than that. When one Monster of the Week turns him and his friends (who do act more like actual teenagers) into elementary school children, the other boys point out that Yumoto didn't change at all.

Western Animation

 * Gene Belcher from Bob's Burgers, combined with Cloud Cuckoolander.
 * Steven from Steven Universe has a Vague Age for the first season and a half, but in "Steven's Birthday" is revealed to be turning fourteen. His Muggle Best Friend and Love Interest, who's not even thirteen but is more mature, is shocked. Aside from the fact that he still sleeps with a teddy bear and generally acts like he's under eleven, Steven also looks quite younger: because his mother was an ageless alien whose species have their physical forms match their mental ages, this ended up applying to Steven as well, with his dad showing Connie a scrapbook showing that Steven hasn't psychically aged since he was eight. Future would finally have a more mature (but emotional unstable) finally look his age (at that point, 16).
 * The Loud House:
 * Downplayed with 11-year-old Lincoln Loud, who mostly acts his age, but he still has a stuffed rabbit named Bun-Bun which he cares for with an affection most kids his age have outgrown.
 * His friend Clyde also has shades of this since he is shown to be a fan of Blarney the Dinosaur, which is meant for a much younger age demographic.
 * Luan is 14 years old yet has a fondness for puns, pranks, clowns, novelty toys, and childlike comedy in general.
 * 13-year-old Lynn is not above throwing tantrums when she doesn't get what she wants and exhibits very poor sportsmanship for a Passionate Sports Girl. Her childishness is also exhibited in "Cereal Offender" where she rides on a shopping cart and wrecks many aisles, and in "Sitting Bull" where she plays on a tire swing in the backyard with her younger twin siblings, Lola and Lana, which her older four sisters deem her immature for and are thus reluctant to hire her as the fifth babysitter.