Fan Casting Improbable Age in Sort by Tropes

A character whose age seems inappropriate with their occupation or position given the amount of experience they would need, whether or not their age is actually important within the storyline. Tends to lean to the younger side, resulting in the Teen Genius, Kid Hero. Sometimes, a disproportionate amount of the (especially female) cast is inexplicably under thirty. In benign cases, it's because the writers assume the audience better identifies with characters their own age.