Age Is Relative

The Competence Zone gets intimate with the Rule of Perception: The physical appearance (and sometimes emotional maturity) of the cast isn't determined as much by their age, but rather their characterization and relative position within the cast. This can seemingly place them outside the normal Competence Zone, until you find out their true age.

In short, the more competent or mature you are, the older you look. And the inverse is true. A character in a position of leadership may look five to ten years older than they really are, while the youngest character will often look, and sometimes act, like a kid. This is especially true with preteen and teenaged characters, for whom their level of maturity is often mirrored by their physical development.

A prime cause of Younger/Older Than They Look. Compare Methuselah Syndrome, Acceptable Targets, Old Master, and Vague Age.

Anime and Manga
"Kaiba: Wait, did your testicles just drop in the last five seconds or something?"
 * Pretty much the entire female cast of Mahou Sensei Negima. Gentle, motherly (but scary when she wants to be, just like a real mother) Chizuru looks to be the third oldest of the class. The first and second, veteran mercenary Mana and wise Ninja-girl Kaede, can't even get their student discount at the movies. Meanwhile, the two least mature members of the class, the Narutaki twins, look younger than their ten-year-old teacher, even though they're about a month older than Chizuru. Another prime example is Chizuru's roommate Natsumi, who's pretty underdeveloped, even goes as far as to use a variant of the "Oneechan" (Big sister) honorific when addressing Chizuru despite being a few months older than her.
 * Negi himself falls under this. Later in the series, he spends a good chunk of time magically aged up a few years...and it fits his personality perfectly. Plenty of people comment that he doesn't really act like a ten year old. He does occasionally subvert it however: He still occasionally smiles like a ten-year-old. It also works in reverse where characters who he has seen naked many many times in the bath (he doesn't like to wash his hair) and other places without a problem suddenly give him the Megaton Punch after a slightly embarrassing situation.
 * The Knight Sabers in Bubblegum Crisis. Primarily in the characters of Linna and Nene, who despite being only a year apart in age, look nearly seven years between them.
 * The best duelists are or look older in Yu-Gi-Oh! (cf. Seto and Dark!Yugi). Yugi himself is a good example, especially in the anime, where his change from his innocent normal self to Dark!Yugi is accompanied by him actually growing taller and more mature-looking.
 * The Abridged Series lampshades it:

"Kyon: Mikuru:"
 * Despite great variations in their appearance, most of the senshi in Sailor Moon are aged within a year of each other, with any grade differences being a product of also being transfer students. This is obscured the fact artists draw Haruka and Michiru as much older looking than the other characters, who gushed about how mature they seemed. The show even poked fun at the idea that Haruka's hobby of car driving doesn't really make legal sense.
 * For those who are unaware Usagi and the Guardian/Inner Senshi are 14 when the show starts (and 15 when the outers are introduced and 16 when the show ends) Uranus and Neptune are 1 year older, while Setsuna is hard to pin due to time travel but is physically in her 20 (And attends collage). Hotaru is twelve years at the time of her introduction but due to age-downs and ups it's hard to pin. Chibiusa is physically around 5 then around ages to around 10. In the manga she's really 900 years old though.
 * Mikuru Asahina in Haruhi Suzumiya. Though one year higher than the rest of the cast, she is often a victim of Haruhi's molestations, though this decreases as the light novels go on.
 * Not to mention that


 * said with a huge smile on her face so it... so it might not be classified.
 * While the more mature characters in One Piece not necessarily look old compared to their age, a case could be made for the two least mature members of the Straw Hat Pirates, Luffy and Chopper. Luffy is 19 and looks and acts like a kid that could be anywhere from 5 to 13 depending on the situation. Chopper is a teenage reindeer who is very naive and does not know much about the world except for medicine, and he looks like a cute little teddy bear in his usual form.
 * Eyeshield 21 plays this trope to ridiculous extremes. Almost all of the cast is aged 15-18, yet you have characters like Komosubi who look and act like they're about 10 and characters like Mr. Don who look older then their own parents.
 * Slam Dunk: Done most obviously with Akagi, Uozumi, and Maki, who are all captains of their basketball team and all look about 30 (atleast). Subverted later on when the very boyish-looking Miyagi's made captain.

Film

 * X-Men:First Class does this in many ways with Mystique. She is about as old as Xavier. But as a shapeshifter, she chooses to look younger, and she prefers to stay with the teenage recruits. And according to Hank, her body ages slower. Then, Magneto dumps her, and states that maybe he will be willing to sleep with her in ten years.
 * In The Godfather Part III Mary Corleone is technically in her mid to late twenties (she's actually older than Michael was at the beginning of the first film). However she is portrayed very much as an innocent, wide eyed teenager (being played by the then 18-year old Sophia Coppala is also a factor.)

Literature

 * In the earlier Harry Potter books, when Ginny was outside the Competence Zone, she was described as "little" and in general made to sound like she was practically still sucking on a pacifier, despite being only one year younger than the trio. This ended after she Took a Level In Badass.

Live Action TV

 * A non-animated example of the emotional maturity part (obviously physical maturity is probably not going to be fudged this way in a live action show): At the end of Buffy the Vampire Slayer, Dawn was the same age Buffy was at the start of the series. Dawn was treated as immature and whiny, while Buffy was treated as justified in her conflict with adults and generally as fairly mature in most matters.
 * While Dawn was quite whiny and downright bratty in seasons 5 and 6, there was a shift in her maturity during season 7. She was certainly as mature as Buffy who shirked her responsibilities and lied to her Watcher, mom and friends so she could go make out.

Real Life

 * It's been said that Presidents of the United States age in dog years while in office. Compare current photos of Barack Obama with those from the 2008 campaign.

Video Games

 * The OG Numbers in Super Robot Wars: Original Generation. Compare BFS-swinger Sanger Zonvolt to XO-slash-Captain Tetsuya. Both are the same age (27), but as Sanger is a battle-hardened veteran, he looks noticeably older than Tetsuya, who over the course of the series is just starting to grow into the role of a capable commander. Another example is fellow captain Lefina Enfield, who looks noticeably older than her nineteen years would suggest.
 * Older?!
 * Tales of Symphonia presents the rare triple subversion. When the characters are introduced to Presea, she's clearly a child, but strikes the characters and remarkably mature and capable (among other things, being responsible for single-handedly maneuvering a giant log that two grown men can't handle). The characters find out later that she's less mature than numb, and once fixed, she seems slightly more childlike. As the game moves on, she's revealed to be and presents herself as being both young and mature. Characters like Raine and Regal are played straight, however, as they seem rather older than their respective ages (mid-twenties, early thirties).
 * Of course, that's also somewhat justified, as Regal is The Atoner, and Raine got a Promotion to Parent when she was eleven.

Western Animation

 * Avatar: The Last Airbender has Sokka explicitly stating that he can't remember his mother without Katara's face, because she has been acting as such since their mother's death. She is his younger sister.