You may be looking for one of the following:
- Absurdly Youthful Mother: A woman who appears to be too young to be the mother of a particular child(ren) (this may be due to Writers Cannot Do Math, or the character really could have been very young when she had children).
- Dawson Casting: High school students in live-action films and TV shows are often played by actors who are old enough to have finished high school.
- Hollywood Old: An actor often tends to look younger than the age their character is supposed to be.
- Improbable Age: A character is unusually or impossibly young to have gotten a certain role, rank, job title etc.
- Underage Casting: When an actor is far too young for the character that they're portraying.
If a direct wick has led you here, please correct the link so that it points to the corresponding article.