Typically, a main or recurring character who regularly interacts with characters noticeably younger, usually resulting in their maturity level regressing to roughly the same level (if not even lower). They're still capable of acting their age... they just usually don't.
This character shows up most often in anything aimed at younger audiences, where they are Played Straight so as to be more appealing to kids. When Played for Laughs, it's to show an immature or silly character.
When examples of this trope show up in shows where they are above the Competence Zone of the rest of the cast, they tend to be teachers, which often yields Sensei-chan.
Contrast with Wise Beyond Their Years, as One of the Kids is rarely treated as any kind of authority, because of their casualness they often do not even earn special respect.
Related to, but not to be confused with, Manchild. Not to be confused with One of the Boys or One of the Girls. See also Friend to All Children.
