When you subtract the age (or apparent age) of a child from that of their parent, this results.
Typically done to justify a hot mom, but sometimes, she really was that young when she gave birth (or the other gender around). Sometimes she only appears this way because of the art style.
In Real Life, a girl can conceive as soon as she has her first menstrual cycle, generally around 11 or 12 years old... although in rare cases this can occur at an even younger age, such as five years old.note Lina Medina, the girl in question is an example of a case of extreme precocious puberty i.e. puberty at an absurdly young age, and reportedly had started getting her period while she was still an infant. She is the youngest mother ever in recorded history, though she was too young to fully remember the pregnancy and her parents raised the baby as her brother. While the girl's father was eventually cleared of rape charges, there's still the Fridge Horror question of who the parents allowed around their daughter. The GuinnessBookOfRecords dropped the "Youngest Ever Mother" category held by Medina out of exactly these ethical considerations; there was also the fear that somebody somewhere was going to try to beat this record.
Teen Pregnancy: Pregnancy at a young age is clearly implied or stated.
Compare and contrast Playing Gertrude, where the actor playing the parent is absurdly youthful although the character may not be.
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Live Action TV
In Only Fools and Horses, Del wins a family holiday for the Trotters by entering a children's art contest with a picture painted by Rodney when he was 14. In order for them to be eligible to claim the prize, Rodney (who's in his early 20s) has to pose as a 14-year-old boy, with Del (who was 16-17 when Rodney was born) pretending to be his father. Del at least has the sense to claim Cassandra, about the same age as Rodney, as a stepmother.
Tracy Jordan's adult son on 30 Rock is far too old. In an aversion, Tracy sees through the deception immediately (while his friends assume the Cloudcuckoolander was actually taken in).
Discussed in the Agents of S.H.I.E.L.D. episode "T.R.A.C.K.S", when Agents Coulson and Simmons are posing as father/daughter family. Simmons comments that Coulson is "far too young to have a daughter my age." (Though it would seem she's just being polite: the official website states that Coulson was 49 and Simmons 26 at the time the mission took place, meaning that they could easily be father and daughter at their respective ages.)
In Avatar: The Last Airbender season 3, Sokka and Katara fake being Aang's parents at a Fire Nation school. The two of them are barely older than he is and wear unconvincing disguises (Sokka wore a fake beard, Katara wore a pillow stuffed under her shirt to make her look pregnant.) to pass themselves off. To really blow some heads, just remember that in terms of pure chronological age, he could be their great-great grandfather.
Cardcaptor Sakura: It is never stated how old Syaoran's mother Yelan is, only that she looks very young and beautiful enough to stand out in a universepopulated by The Beautiful Elite. She is, however, thought to be somewhere in her mid-to-late 30's, and possibly married as a teenager. This is never confirmed, because she is only seen in the first OVA.
As mentioned above, in the dub of Digimon Tamers, Ruki/Rika's age was altered, but the age of her mother was left alone. Ruki is implied to have been the result of a teen pregnancy in the original. The dub's numbers indicate that Rumiko gave birth at thirteen. Oops.
Kill Killer features a story with a "Dirty Callahan"-like killer of pedophiles who, after a lengthy period in prison for beating the supposed killer of his five year old child, reunites with his partner, the mother of the child... a 19 year old teenager. Someone obviously couldn't make a subtraction...note While children giving birth around the age of 14 can be Truth in Television, it is fortunately exceptional.
Films - Live Action
In The Haunted Mansion, Marsha Thomason plays Sara, a mother whose daughter and son are aged 13 and 10, and who is implied to be around the same age as her husband, who is played by Eddie Murphy and obviously meant to be in his late thirties/early forties. In reality, she is less than fourteen years older than the actors who portrayed her children, and was clearly only in her mid-twenties when principal photography took place.
To a lesser extent, Harry's parents ended up having Harry when they were 20. Rowling acknowledges this and wishes she could go back and fix it.
Live Action TV
In One Life to Live, Greg is explicitly stated (in courtroom testimony, no less) to have ten years' experience as a fully licensed practicing neurosurgeon, putting him in his early 40s at the youngest. His mother is played by an actress in her late 40s, and to make things interesting he has a 15-year-old sister. All the characters were introduced during 2009 so SORAS isn't involved.
Supernatural. John Winchester may be ruggedly handsome with a beard, but this hardly hides the fact that he looks MAYBE ten years Dean's senior. The same is true of his wife Mary when she appears in an alternate reality where she is still alive in the present day, played by the same actress who plays her in flashbacks to the brothers' childhood (although this one has an in-universe explanation in that it is a Lotus-Eater Machine fantasy created from Dean's memories of her).
In the third episode of Girl Meets World, Topanga tells a small child that she's 30. Topanga and Cory have a daughter, Riley, in 7th grade. If Riley is normal age for her grade (12-13), Topanga had her when she was 17-18. Given that Boy Meets World goes through Topanga's second year of college and she had no children up until that time, clearly someone didn't do the math.
That or she was simply lying about her age.
Dead or Alive: Ayame, the mother of Hayate, Kasumi, and Ayane. She's had three kids, Hayate being in his early twenties and Kasumi and Ayane being in their late teens, and in her every appearance so far, she doesn't appear to be much older than Kasumi.
In The Sims 3, Boyd and Susan Wainwright are 11 days until Elder while playing in normal lifespan (AKA they're 62 Sim Days old), while their daughter, Blair, is in her very first day of Young Adult (31 Sim Days old), meaning that Blair was born while both her parents were in their very first day of Young Adult, and thus she would have been conceived when her parents were teens in "normal gameplay". In the meanwhile (a completely different family\household but still in Sunset Valley) Leighton Sekemoto is 19 days until Adult (basically middle-aged) while his son, Sam, is on his first day of toddler. This is because, while 1 Sim Day = 1 Year in the Child, Young Adult, Adult, and Elder Stage, it equals 6 months in the Baby, Toddler, and Teen lifestage.
Homestuck. Bro Strider. Most fans think of Bro as being in his early/middle 20s, until you sit down and do the math. He was at least an adult - 18 - when he adopted baby!Dave. Dave is thirteen now, making bro at least 31. The same might apply to Mama Lalonde and Dad Egbert, but Mom and Dad seem to be thought of as "older" than Bro, so I don't know. The whole lot of them don't seem to change in appearance between adopting their children and thirteen years later.
The Egbert Family. Nana was born in 1910. John was "born" in 1996. Dad appears to be a regular middle aged man, but if Nana had Dad around age 20 (as was normal for a woman of her time period) Dad would have to be in his sixties when John is "born." If Dad is about, say, 40 when John is 13, Nana would have had to have given birth to him in HER fifties-sixties. It's unclear how old Dad is SUPPOSED to be, but so far it doesn't seem to add up when considering Nana's and John's ages.
Anime and Manga
Diva from Blood+ looks, sounds and even acts very youthful, if not childish and immature. She becomes this after she rapes and kills Riku., who happens to be 13 years old. She does mature slightly a year after the Time Skip.
In Sword Art Online, Kirito and Asuna adopt the AI Yui as their daughter. At the time, Kirito and Asuna are 16 and 17, respectively. Yui, by virtue of being an AI appears to be 8 despite in fact being 2.
Twilight adopts the magically generated Nyx. Biologically, they're a late teennote specifically, the pony equivalent of a college student and child respectively, and that's not even going into the newly-made or millennium-old question with Nyx. And things get even weirder when Nyx is artificially aged up to an adult horse...
Fan works disagree on whether Twilight is Spike's mother figure or big sister figure. Today she's old enough to be considered a mother figure, indeed the equivalent of a woman in her late teens or early twenties, but many forget that Twilight hatched Spike when she was the equivalent of a third grader. If she is a mother to him, that makes her, well, an absurdly youthful one.