In Code Lyoko, Aelita is chronologically 22-23, but ten of those years were spent in stasis as a Digital Avatar in Lyoko. Because of this, she has far more knowledge and intelligence than any ordinary teenager, but she's still physically and emotionally 12-13, and is in a relationship with Jérémie, who's physically the same age.
Judging by the age of her daughter, Susan Vandom of W.I.T.C.H. is about 45 years old and works in IT. Judging by appearances she's about 25 and a professional glamor model. Compared to her, Cornelia's mom and Irma's mom are less mind-blowing, but still look nowhere near what middle-aged women should (or do) look like. Though we do get an idea where their daughters get it from...
Halinor, the blonde bombshell of the previous team of Guardians, still looks quite good for her age, with only a few facial wrinkles. She definitely doesn't look like she's pushing 60, and Yan Lin even comments on Halinor's youthful appearance when the two see one another in a second season episode. Similarly, when we meet Kadma (basically a grown up version of the Alpha Bitch) she's supposed to be at least in her sixties, maybe seventies. Physically she has, like, a line under her eyes and white hair...otherwise she maintained her prized assets and slender, youthful looks perfectly. Compared to her contemporaries, Yan Lin (who looks her age) and Narissa (who looks the Universe's age) one has to assume she's been either getting very good plastic surgery over the years or tapping her magic to stay young.
Batman: The Animated Series has Mary "Baby Doll" Dahl, a former actress-turned-criminal who, due to a rare genetic disorder, continued to look like a small child even after reaching adulthood. Her anguish over being an adult trapped in such a body, and the public's refusal to respect and treat her like her true age, is the source of her criminal mindset. In a later episode, "Love is a Croc", she romantically teams up with Killer Croc, a similarly outcast bruiser who looks like a human crocodile.
Kearney Zzyzwich (the bald, fat bully with the white T-shirt, blue pants, and studded wristbands): Even though he looks old enough to be in high school (yet he's been held back in the sixth grade for a long time, and Nancy Cartwright (Kearney's voice actress) established that Kearney is actually 14 years old), Kearney is actually older than that. He was alive when the Watergate scandal and the 1976 Bicentennial happened ["Lisa the Iconoclast"] (which would make him around 22 or 23 years old), has a child from a divorce ["A Milhouse Divided"], was in Otto the bus driver's third grade class ["The Mook, The Chef, The Wife, and Her Homer"], is old enough to drink in a bar ["El Viaje Misterioso de Nuestro Jomer"] (though he often uses fake IDs ["Much Apu about Nothing"] or tricks Homer into getting him alcohol ["Last Tap Dance in Springfield" and that Mary Poppins parody episode from season 8]), was tried as an adult and put in an actual prison ["Realty Bites"], voted in a U.S. Presidential Election ["Treehouse of Horror XIX"], pays taxes ["Lisa Simpson, This Isn't Your Life"], and has a car ["Lisa's Date with Density"], even though he can be seen on the school bus. Of course, this can be interpreted as a satire on how children grow up too fast and how the most childish of men are somehow fathers before they're emotionally ready (it is The Simpsons after all) or, as mentioned before, yet another sign that the show does not want their characters to age.
Ned Flanders: All outside appearances seem to imply that Flanders is around the same age as Homer (late 30s/early 40s), but in season 10's "Viva Ned Flanders", Flanders reveals that he's sixty years old and looks young because he's never done anything sinful and crazy in his life.
Krusty the Clown appears to be the same age as most of the prominent adults on the show, but it's implied that he's actually much older. Considering his harsh lifestyle (and the fact that his father is still alive and well), this is pretty impressive. Even more impressive when you remember the episode where Krusty, while taking a summer hiatus, puts on a rerun of his show from 1961 (which is in black and white). And the scenes from his childhood (in the episode where he first reveals his Jewish roots and Bart and Lisa work together to reunite Krusty with his estranged father) look like they're from the 1920s or '30s at the latest.
Quagmire from Family Guy has a driver's license that indicates he's 61. His secret is carrots.
Speaking of Quagmire, Quagmire's transsexual father from the episode "Quagmire's Dad" barely looks any older than her son. Quite impressive, given that she would have to be in her seventies or eighties.
In an episode of The Marvelous Misadventures of Flapjack, Flapjack encounters what appears to be a creepy, annoying, wide eyed little boy who won't leave him alone. Finally he tells him "You need to grow up", and the "boy" responds "Grow up? I'm 38 years old!", which is followed by a Gross-Up Close-Up of his wrinkled face.
Probably Mrs. Dr. Anne Possible. In fact, in one episode of Kim Possible ("Mother's Day"), Dr. Drakken truthfully believes that she's Kim's sister.
"Is he hitting on me?"
Granny Crocket after extensive surgery.
Granny Crocket: "Granny had a little work done."
Slurms Mackenzie's party girls from Futurama. Before he makes his Heroic Sacrifice, he says that they have served him well during the past 40 years. The two of them look to be about twenty years old.
Bender accuses the Planet Express ship of this because the ship has tubes.
And Fry is 1000 years older than he looks.
In one episode the entire crew jumped into an alien fountain of youth to reverse their rapid anti-aging and as a consequence all came out physically younger than their original ages (except for the Professor, who got even older). Which explains how after over ten years they all still look roughly the same age.
Magneto in X-Men: Evolution is at least as old as his movie counterpart, but looks only middle-aged the first time we see him without his helmet. In a later episode it turns out he uses his knowledge of genetics to maintain vitality and the appearance of youth, and when this fails him he reverts to his true age. Then he uses the Rebirth formula (which granted Captain America his powers) to de-age himself again, and looks like a young white haired man for the remainder of the show (on the rare occasions he's seen without his face-concealing helmet).
Thanks to Wolverine's self-regeneration power, his true age will never be revealed but it's shown he was at least old enough to have served the military along with Cap, which he did in this cartoon. Magneto was a kid back then.
Mrs. Ridgemount, Lo's mom, from Stoked. Let's give an example: look at this◊ picture. One of these women is a middle-aged woman with three kids, the other is a girl in her late teens. Guess which is which.
Mr. Garcia, Jonesy's father, from 6teen. He looks more as the older brother of his children. Heck, he looks younger that his new wife, Emma Masterson, Jen and Courtney's mother.
The young adult male Smurfs are really about 100 years old (150 in the cartoon show), while Papa Smurf is 542. Of course, this could also be referred to as the Vague Age where it's uncertain what particular human age their ages are supposed to correspond to.
Their human friends Selwyn and Tallulah are more than 200 years old, according to a Season 8 episode.
Miss Martian in Young Justice is actually 48 years old in Earth years, but since Martians age slower than humans, she has the mentality and maturity of a 16-year old.
Superboy is a bizarre example - he's technically Younger Than He Looks (being that he is chronologically 5 1/2 years old by the second season) but is mentally 21 in the body of a 16-year-old. It's stated that due to the cloning process, he will look 16 for the rest of his life.
Arsenal is in his 20's, but is physically around 15 years old since he was cryogenically frozen for eight years.
Cute, pink-haired assassin Claire from Titan Maximum looks like she's eight, but Gibbs points out she's 24.
There was a clever variation in Courage the Cowardly Dog with the episode "Conway the Contaminationist", in which the eponymous Conway is in his 130s and can backflip around at will if he feels good—but, instead of looking distinctly elderly, he just looks bizarrely deformed.
Buffy Shellhammer from "Poor Little Rich Turtle", she states that she's fourteen but she look and acts like she's about 8-10 years old.
Strawberry Shortcake, and her similarly aged friends, are implied to be this in the 2003 incarnation. She ages noticeably through the series yet never appears older than thirteen. This is despite the fact her sister aged from infancy to at least six years old, she can drive a car, and she even moves out. Her 2009 counterpart probably applies too, looking like a teenager but being able to own her own store.
Transformers Animated: Ratchet is physically the oldest of the Main cast, with Bumblebee, Bulkhead, and Optimus being built after the war, however, later episodes reveal that Prowl was a draft dodger back in the war, and he looks about as old as Oprimus, he spent centuries meditating on a rock, so perhaps that slowed his aging process. Arcee doesn't look old either, but, unlike Ratchet, she was in medical care after she met him, so likely that's why she didn't age.
Apparently, according to the two-parter Superjail! season 1 finale "Time Police", the Warden is actually chronologically in his hundred-teens despite looking to be in his late 20s and early 30s!
Played with between Pinkie Pie and Fluttershy. Many might assume Fluttershy to be the youngest of the Mane Six given her shy demeanor and childlike innocence (though Pinkie is also very childish), and Pinkie even jokingly tells Fluttershy in season one to think of her as "auntie"... but Fluttershy replies, a bit petulantly, that she's actually a year older than Pinkie Pie. In the flashback to their youth, Fluttershy looks taller and more lanky/gawky than the rest of the cast, indicating she may actually be the oldest.
Strawberry Sweetcake of Drawn Togetherwas the smallest ass they could legally tap, after all.
Implied with Ms. Keane and Professor Utonimum from The Powerpuff Girls. The series takes place in The Nineties but they go back to their childhood in The Fifties. That means they're around fifty to sixty, while they don't look any older than their mid 30s.
Bobby Hill of King of the Hill even though he's 13-14 years old in later seasons, due to his childish appearance, short stature, and immaturity you'd think he's around 10.
In the Adventure Time episode, "The Vault," Finn sees a vision of his past life, a girl named Shoko, who encountered Princess Bubblegum during the early days of the Candy Kingdom. Bubblegum looks exactly the same then as she does now, suggesting she is far older than the nineteen she claims. She only laughs nervously when Finn points out this discrepancy at the end of the episode.
Cubert Farnsworth from Futurama is around 13 but looks and acts like he's around 9.
Winx Club has Miele, the younger sister of one of the main protagonists. When she first appears, she looks like she could be no older than a child in the early grades of elementary school. Three seasons later, she reappears and looks like she's aged ten years!
Codename: Kids Next Door one-shot villain Mr. B looks like a baby, and calling him one is his Berserk Button. In his only episode, he tries using the KND's satellite network to fire an age-changing ray down on everyone to turn everyone into babies, so that no one will be able to call him one.