The Saiyans: By the time they're in their twenties, they still look like teens or preteens, and it isn't until well later on in the series that they're looking like adults (though Vegeta goes through this relatively quickly). Even to the degree that in Dragon Ball GT/Late Dragon Ball Z it gets highlighted by Vegeta, saying that he might be in his 50s, but he ages slower than the rest of the cast. Bulma does not enjoy this.
Vegeta's brother Tarble is probably the best example of this as far as Saiyans go- he looks like he could be in his teens despite being older than Goku, who was around 40-years-old when they met.
In the original Dragon Ball Goku looks around seven when he's twelve, and at age fifteen he looks more-or-less the same. It's not until age eighteen that he finally starts to look like a teenager.
Mrs. Briefs, Bulma's mother, doesn't seem to get any older over the entire course of the series. By the Buu Saga, her daughter is over 40, and she looks the same as she did when she'd first appeared roughly 25 years earlier.
Bulma also qualifies in the Buu Saga. At that point she's in her early 40s, but could still easily pass for late 20s/early 30s.
And for those starting out with Dragon Ball Z back in the mid-90's, Bulma looks like she is in her high school years, while Krillin looks like he's in his middle school years. They're both well into their 20s.
And then there's Chiaotzu, who looks like a 10-year old child through his entire life. By his last appearance in Z, he's 46-years-old, just one year younger than Goku, and he still looks like a little kid. This is kind of explained in that his original design was based on a traditional Chinese ghost.
A non-human/Saiyan example might be Frieza. Between flashbacks to the Bardock Special, which occurred at least twenty+ years ago, to the present, he hasn't visibly aged a bit. He could easily be Really 700 Years Old, but his age is never given.
Krillin, who has incredibly low height, does not look his age throughout Dragon Ball Z. Neither does his wife 18, whose modified body ages very slowly.
The Pilaf Gang - Emperor Pilaf, Shu and Mai - are all chronologically older than Bulma, likely by several years. However, thanks to a badly worded wish, they were given child bodies whilst their adult minds and memories remain.
In Part II, Joseph's brain nearly breaks trying to comprehend how Lisa Lisa could be fifty years old. The answer, incidentally, is the Ripple. Supernatural powers in this series slow aging.
In part IV, Koichi Hirose starts out at 16 years old, but one could easily mistake him for an elementary schooler due to his short height and his Cross-Dressing Voices
Funnily enough, Hirohiko Araki himself is known for the fact that he never seems to age, allowing him to still look like he's in his 20's for several decades. In a way, it seems almost inevitable that his characters would carry on this trend.
A very minor age-difference with Koruri from 7 Seeds. She's one of the shorter characters, without much development and her curly hair makes her seem even more youthful. She is 17 years old, but looks about 13 years old. The other teams are surprised to learn that she's the same age as the other members of her team, especially her boyfriend, who is a year younger than she.
Faye Valentine from Cowboy Bebop is much older than she appears, having been cryogenically frozen for 54 years while recovering from injuries she sustained on a space shuttle accident (possibly one of the first public space shuttles).
The Familiar of Zero: Saito Hiraga looks no more than 13, but is in fact a 17-year old. Even worse is his Tsundere mistress, Louise, who only looks ten at the most and is the same age as Saito.
Libra Dhoko from Saint Seiya fills both this and the Younger Than They Look trope (oddly enough). He's over 200 years old, with a body of a 21-year old (after shedding his old skin) OR a 21-year old in a very old body.
To elaborate more: Dohko learned a technique that allows him to slow down his metabolism and retreat himself into an severely aged and weaker body, but with extremely expanded lifespan. Anytime he stops using said technique, he returns to the age he had before activating it.
Mika-sensei in Doki Doki School Hours is a 27-year old woman, though she appears to be ten years old. This doesn't prevent one of her students, Kitagawa, from being attracted to her. In fact, it encourages it.
Levi from is this according to Shrug of God. His exact age is unknown, other than being this trope almost word for word.Word of God is that he's over 30.
Ymir is an unusual example. She was trapped in Titan form for 60 years, and did not age a single day during that time. As such, she is technically in her late 70s but looks to be in her late teens.
In Please Teacher!, Kei is a 18-year old male lead who is functionally fifteen years old due to a coma that resembled suspended animation. Among other things, this also served as Lampshade Hanging to explain the contrivance of him and Mizuho shacking up together, since eighteen is minimum age to marry... And in a plot twist late in the story, he finds out that the Little Miss Snarker in his circle of friends has the exact same condition, and is actually in her twenties.
Minnie-May Hopkins from Gunsmith Cats took "growth retarding" drugs in order to appeal to a certain type of clientele in a brothel. However, this is supposed to be squicky to the viewers. It squicked out the American editors so much they reversed it in their translation so she's taking drugs to counter an unnamed medical condition. In the English version, May had tried to make herself look older than she was in childhood in order to not squick her adult boyfriend so much (he was Lolicon, but understandably had serious issues about the morality of their relationship) but it backfired, leaving her first looking older than she was, but not growing an inch since.
Somewhat related cross-cultural example: when Digimon Tamers was translated for Western network television, they raised the kids' official ages by a few years (from ten to thirteen), possibly due to what happens to them and the relations they get into squicking the translators a bit. Digimon Adventure 02 and Digimon Frontier also had all the kids ages raised a year.
The same thing happened with 4Kids' version of Tokyo Mew Mew, presumably for similar reasons; an especially jarring example is Bu-Ling, who begins the series at eight years old, yet the dubbed version managed to make her fourteen. Oddly, Tokyo Pop's manga translation moved the characters down a year to appeal more to a young audience.
Aur the protagonist from Maou no Hajimekata was an old man at the end of his lifespan by the time he finished his mana potion, rejuvenating himself was the first thing Aur did with his new immense power. Also the many demons and other races seen in the series.
The main character of Shibatora is a 22 year old police detective, but is frequently mistaken for a middle school student.
The title character of Kamichama Karin, as well as her True Companions, are all in the seventh grade, but look to be no older than seven, while other characters such as Kirio Karasuma and Kirika Karasuma look to be around sixteen, even though they are only one year older than the main cast. This is more Koge-Donbo's style than anything else. In one panel in the first chapter, Karin is shown drawn as her true age; for almost all of the rest of the series, she and her classmates are drawn as chibis. The same goes for the author's other series Pita-Ten.
Hagu from Honey and Clover is apparently 18 at the begining of the series, despite looking and acting like an 8-year old — which doesn't keep some guys from falling in love with her. Which is made worse by the fact that it is openly stated she stopped growing at six and she barely changes by the time she's twenty.
Galaxy Angel has Mint, a 15-year old who looks like she's ten.
Subverted in Mai-HiME (which does in fact include a character fitting this trope, just not that one):
Natsuki: How old do you think I am? Mai:(confused) Aren't you a high school girl? Natsuki:(annoyed) That's not what I meant. I'm seventeen. I was away from school for a year.
Presumably the first Club President of Genshiken, who later turns out to have been the only Club President the Society for the Study of Modern Visual Culture had ever had until he handed it off to Madarame. The members briefly speculated how old the Prez actually was after finding manga books in the clubroom that date back to 1987.
Done in Haruhi Suzumiya with Mikuru Asahina, who is described in the series as "Lolita-looking". She's seventeen, while the rest of the cast is sixteen. She also happens to be Ms. Fanservice and have Gag Boobs, yet still manages this title. Then there's the whole Time Travel thing (and her playful refusal to answer any of Kyon's inquiries about the matter), which makes her real age pretty much impossible to determine. Given the possibilities of future medicine, she could be in her thirties for all we know.
In Welcome to the N.H.K., one of Satou's "study materials" (i.e. eroge) has a cover with a big-eyed, small-chested girl... and the title of the game is actually "I Am Not Loli!"
There's a rather odd version of this in Zettai Karen Children where the Evil Mentor who looks like a middle school student is really in his 80s. This changes his stated intent to "groom" the titular 10-year olds from weird to squicky.
Let's not forget also the said evil mentor's childhood friend and Good Counterpart, three years older Fujiko, who looks and behaves like a horny teenage Kawaiiko, but notwithstanding her looks the titular "children" characters often call her "granny".
In the Eureka Seven manga, Matthieu tells Renton that "the thing about Eureka is... she may not look it, but she's actually 30!" much to Renton's shock. This is revealed to be a lie simply to get Renton's goat the very next panel.
In Eureka Seven Ao, Eureka is a good deal older than she was in the original series (it's more complicated than that due to time travel hijinks), but still looks roughly the same as she did in the past (mostly because she isn't human). She does pull off an older look, though, by way of clothing choices and hair styles.
Asuna Kagurazaka, who looks and is assumed by the people around her to be 14. It isn't clear just how old she is, but flashback chapters later in the manga have confirmed her to have traveled as a little girl with Nagi's company — twenty years ago (it turns out she's over 100 years old). It should be noted that Takamichi, Asuna's adult teacher who also traveled with Nagi, was also a little kid at the same time.
It's hinted that Takamichi spent a lot of time training in Evangeline's time condenser, and therefore his actual age may not match his chronological age.
Also, Fate Averruncus and Zect Filius, the two white-haired pretty boys: While Fate apparently bathed in some Fountain of Youth, Zect is referenced as a "man" despite looking way younger than "Just a little brat" Nagi. And nobody finds this weird, so it may be a racial trait or something.
Then we've got the normal (we think) girls like the Narutaki twins, who look ten years old despite being around fifteen like the rest of the class. They're actually the oldest, other than the vampire and ghost. Likewise, the oldest-looking girl (Chizuru) is actually the youngest.
Major Kusanagi is age 44 at the time she meets the Puppet Master/the Pupetteer and a few years younger in Stand Alone Complex, but her almost completely artificial body looks noticeably younger.
And in one episode there's a female Russian assassin with a similar cybernetic body, who is said to be over 80.
All the demons in Chrono Crusade fall under this, but Chrono in particular. Thanks to a need to conserve his spiritual energy, his typical form makes him look like twelve years old, and even in his true form he looks like he's in his early twenties. However, there's at least about 60 years that he's confirmed to have lived, and it's likely he's much, much older. His former comrades, the sinners, all appear to be in their late teens or early twenties, but they're all near the same age.
There's also Father Remington (who appears to be in his early thirties at the latest but is seen dueling Chrono half a century prior in a flashback).
In the manga finale Fiore/Floretta and Satella (due to becoming Human Popsicles for 75 years).
The extreme cases fell on Konata, Kanata and Yutaka — their heights are 1.42 m, 1.43 m and 1.38 m respectivelynote For those of you less familiar with metric, that's 4'8", 4'8¼", and 4'6" respectively. The relation between this trope and Lolicon has been thoroughly explored: Kanata is the (deceased) wife of Soujiro, and Konata wore School Swimsuit to attract "people with special interests" in the Beach Episode, and when Yutaka lamented her lack of boyfriend due to being an illfauxli, Konata told her that a lot of people are looking for her, should she not choose... Yutaka's demeanor, and the fact that she and her interactions with Minami seem verysimilar to another yonkoma-based anime with anactualchild don't help her case—even the mangaka shouted that out when Yutaka introduced herself in front of the class, and gave her pigtails.
Cinque looks like she's around ten, but she's one of the oldest members of the Numbers Cyborgs, and is officially the same age as Teana.
Fate's familiar, Arf, in the third season when a ten year Time Skip means she actually is old enough to appear as a grown woman... and she looks like a child in order to conserve energy. Before that, she was Younger Than They Look.
And in a less extreme example (also from the first two seasons), Chrono Harlaown looks to be roughly nine or ten years old, just like Nanoha, Yuuno, Fate, etc. In reality, he's fourteen.
Lutecia: Even though we've got one shrimp, the three of us are of the same age. Caro: What did you say!? But I've grown taller by 1.5cm!
Precia Testarossa, the Big Bad of the original series, looks like she's in her 20s. She's 54.
Likewise, by the time of StrikerS, the now 41-year-old Lindy Harlaown doesn't appear to have aged a day in the 10 years since her first appearance. And for that matter, in the original series she was 31 and looked like she was in her early 20s.
Ditto for Momoko Takamachi, who still looks like she's in her twenties during her cameo in ViVid, despite being 47. Coincidentally, all three of them are Vivio's adoptive grandmothers.
In the alternate continuity of Magical Girl Lyrical Nanoha INNOCENT, Alicia Testarossa is several years older than her 10-year-old sister Fate, yet she still looks like the same 5 year old she was in the main series.
Kōran, the heroine of Mikado no Shihō looks much younger than her actual age of eighteen, causing her to repeatedlyremind everybody else that no, she is not a child. And then there is tall and handsome Shiki who is really seventeen, younger than the heroine herself.
Also L, who can blend in with a group of teenagers in his mid 20's.
Many don't realize that Misa is older than Light. The folks at Viz Media even make this mistake in one of the DVD commentaries. Justified in that her young looks are accompanied by extremely childish behavior.
Tsubasa Shibahime from Kare Kano is a high schooler like the rest of the cast, but looks like she's twelve.
Mitsukuni "Honey" Haninozuka from Ouran High School Host Club looks and acts like a 10-year old to appeal to the Shotacon crowd, but is really 17/18 years old, and therefore, the oldest member of the club. At one point he infiltrates the elementary school section of Ouran just by wearing his old uniform.
Thorkell from Vinland Saga is a more realistic example than most but he still in no way looks like he's in his fifties. Even more notable because he is an enormous man whose facial features suggest that he suffers from gigantism, which should mean that at that age he should be at death's door, already.
Azuma and Seira Umeda from the manga series Hana-Kimi. They both look pretty young despite the fact that Seira is fifty-one and Azuma is fifty-four.
The art style of Hidamari Sketch made everyone like this, but Yuno needs some specific mention. Her extreme proneness to childlike mood swings/general childish behavior and her 1.44m height made her seem more like seven than fifteen. Actually part of this series' premise was on her trying to grow out of this.
A Certain Magical Index: Komoe-sensei looks very young (no older than ten) and wears bunny pajamas to bed. Yet she is an adult, and her apartment is littered with beer cans and cigarettes. Quite a contrast there. For the longest time, there was no explanation for this, but in later volumes rumors eventually surface that she was the subject of some kind of age-regression experiment (though whenever anyone mentions this in front of her, she denies it).
However, in the Natural Selector Arc, there's Oumi Shuri, a Kouga Kunochi who is over 30 years old, but at some time in her life, came upon a disease, probably the Clostridium botulinum bacteria, that stopped her from growing, and maintained her skin and preserved her childish face, and then she had her body modified to the point that she looks around 10 years old.
Yun-Ook Jang, the main character in Forest of Gray City is a 25-year old who keeps getting mistaken for a teenager.
Double Arts pulled this on one of the main characters. She goes to meet a new found friend's family, and is introduced to a little girl she assumes to be his little sister. But wait! That's his mom!
A very similar situation to the one above happens in the manga Code:Breaker, although this time with the female protagonist's mom, and the mom being a bit more endowed.
Rurouni Kenshin: Kaoru refuses to believe that the youthful-looking Kenshin is a whopping 28 years old, although that doesn't stop them from getting together in the end—even though Kaoru is only 17. Likewise, when Yahiko and Misao discover that Hiko is a remarkably well-preserved 43, they wonder if it's a side-effect of the Hiten Mitsurugi style.
And speaking of Misao, she's 16, but looks quite a bit younger, something that she doesn't react well to. One particular filler episode had all the female characters poke fun at this, even innocent little Tsubame, who is shocked that Misao is that much older than her.
Hinako Ninomiya has the appearance of an innocent child though she is actually much older. When she drains the battle auras of "delinquents", she reverts to her true age and becomes a buxom woman in her late twenties.
Also Lukkosai, who is the same age as Happosai but is cursed to turn into a little boy when splashed with cold water.
The manga Cardcaptor Sakura's Eriol Hiiragizawa is actually the same age as Sakura's father, despite looking to be around the same age as her because he magically halted his aging process.
Tsubasa -RESERVoir CHRoNiCLE-: Syaoran is subjectively around 21 when the series begins (Time Travel is involved). He looks like a young teenager. The clones don't look any older than he is at the end, but both of them are in their early-to-mid 30's... yes, the clones are older than their originals. It's complicated.
Fai looks to be in his twenties, but isn't. His real age is never said, but Yuko commented that despite his apparent youth he had already lived several lifespans.
×××HOLiC: Watanuki takes over Yuuko's shop after her death, he goes into a similar un-aging and anomaly-to-the-universe state. By the end of the series he's over 100 years old but physically looks barely older than the teenager he was when the series started.
Kidou Tenshi Angelic Layer's Misaki Suzuhara is of 1st year middle school age. Yet she gets mistaken for much younger, at one point elementary 3rd year, much to her shock.
God from Maria†Holic came upon a long rotted love letter given to her years before, which looked to be pre-War Japan. She looks no different than she does in the present, and since she looks to have been the dorm leader even then, she would have to be at least 70 or 80. Also, Yonakuni.
In Mahoromatic, Suguru's teacher is shown to look exactly the same at the series end as at its start; the series ended after a twenty year Time Skip where she considered Suguru too old at the ripe age of thirty four. This kinda begs the question; how old was she at the series start?
Two of the Sannin have this; Tsunade uses a transformation jutsu to keep herself looking like she's in her late twenties or early thirties (however, the use of her other jutsu makes her appearance without it appear even older than she really is). Orochimaru's body-switching jutsu causes him too look like he's still in his early-to-mid-thirties. Both are between 50 and 53 years old. Jiraiya presumably doesn't use any jutsu to keep himself looking young, as he is the only one who looks his age.
In the third movie, Sakura fights a female assassin named Karenbana who appears to be about 12. When Sakura says to her, "You wear too much perfume for a kid", she replies with, "Kid? I'm twenty-two!" to which Sakura replies, "Oops!"
Sasori is 35 years old. But because he modified his body to become an unaging puppet, his physical appearance is that of a 15-year old.
The First Hokage lived long enough to have grandchildren. Nevertheless, neither him nor his brother the Second Hokage look like they've aged beyond their early 40s; for all we know, they could have been relatively young when the grandkids were born.
When they meet, Naruto instantly assumes that Yagura is a child who never had the chance to properly experience life. He's actually both an adult (though his age isn't given) and the Fourth Mizukage to boot.
Alicia is 19 years old and pretty much looks that way, which doesn't match well with her supposed long experience as an undine. She lives on Mars though, where the years last almost twice as long as earth's. Do the math.
President Aria. 20 years have passed and he doesn't look any different, unlike Grandma, who has considerably shortened.
Played even straighter by Albert Pitt, who, while one of the eldest members of the main cast, looks like a cute little boy due to his working conditions.
Loco from MÄR is thirty-two years old, despite looking like she's somewhere around ten. This is because the Darkness Arms she specializes in de-age her every time she uses them. At one point she ends up de-aging herself into infancy.
In Black Cat, because of being shot with a nanomachine Lucifer bullet, Train transformed into a kid (that looks around 7-8 years old), even though he's actually 23.
Ranewater Calder from Zombie Powder, leader of the Ash Daughter Gang. Although he looks like a young boy, is actually an old man who manages to retain his youthful looks with Phoenix-Samin. His nickname is actually "Old Man Calder".
Mikako Koda from Paradise Kiss is around fifteen years older than her sister Miwako, and they both look like twins.
In the anime of Venus Versus Virus, twins Lola and Layla don't look much older than middle schoolers, but they're actually much older than the teenagers Lucia and Sumire as they're ageless dolls. In the original manga, they're just normal humans.
Break and Sharon from Pandora Hearts don't age and are at least ten years older than they appear. Break is around 120 years old, though he looks like he's in his twenties. Sharon looks 13, but is really 23. There's also Zwei. Another example is Rufus Barma, who appears to be in his twenties or thirties despite actually being over seventy years old.
Given the bizarre way that time works in the Abyss and that most of the characters are contractors, most of the cast actually falls into this. Apart from the above examples, there's Oz, who is 25 though he still looks 15, and Vincent and Gil, who look in their early twenties, were eight and nine years old at the Tragedy of Sablier... a hundred years ago. The Baskervilles also apply as they have been seen in flashbacks over one hundred years ago.And Alice, for that matter. Interestingly, when the lack of aging from first brought up, Break had said that it was a 'possible' side-affect from entering in a contract. So far every contracter has been proven to be older than they look, it makes you wonder why they bothered adding in the 'possibly'.
Now in all likelyhood this applies to Riem also due to his contract witht the March Hare.
In School Rumble, Yakumo is calm, mature, and the object of Hanai's affection. Tenma is short, acts childish, and has Expressive Hair. Try and guess which one is older.
Seguchi Tohma owns this trope. He's 32 but looks 16. In the flashbacks where he's supposed to be 16, he looks twelve. Same problem appears with a bunch of other characters in Gravitation as well.
Yuuki of Saki. First year high school student who plays the role of Token Mini-Moe since she still looks like a kid. Koromo is even worse, she's manages to look even younger than Yuuki despite being one year older, to the point at which a security guard mistakes her for a little kid, rather than a high school mahjong player. In fact, Koromo's the oldest member of the Ryuumonbuchi team, albeit by a small margin, since they're all second-years.
Similar to Koromo, Kurumi of Miyamori is quite short and is occasionally childish, but she is, like her teammates (which includes the incredibly tallToyone), a third-year in high school.
Hayari Mizuhara, a pro player, is 28, something that is repeatedly pointed out every time her name is shown, despite (or because) of how she has a youthful-looking face that hasn't changed much since she was 11.
In NEEDLESS, Disc is a half-cyborg from the time before the big war. This makes her over 100 years old, despite her appearance as the Token Mini-Moe. When this is pointed out in episode 7, Adam is less than pleased.
The age of Road is revealed in chapter 187 to be over 35. What isn't known is whether the young form we see is her real body, or an illusion; for example, although we know she is human, we've also seen her in the form of a doll.
Chivalrous Pervert Cross Marian is a big case as well. He's been allied with Neah for over 35 years, Winters and Klaud are forty and thirty-five respectively, so Cross has been in the war since they were children. In an extra, Komui's Talk Corner #3, the characters debate Cross' age. Johnny says he looks to be in his late twenties, but that he has to be at least forty. Tyki says he's making himself look scarily young. Wisely then asks Allen-who says his master hasn't aged a year since he met him. He then says it's scary, after calling Cross a monster. Hoshino herself, keeps his age hidden in the character info.
Shown in chapter 215 to be true of Allen as well. The Fourteenth is shown wondering why Allen's body hadn't aged in the last 35 years and comments that it has in fact gotten younger.
Yellow from Pokémon Adventures. All the Johto Dex Holders have a hard time believing that someone a foot shorter than them is in fact a year older.
Magical Pokémon Journey has Ginger, who introduces herself as Coconut's sister. She appears to be around 16, which makes her overtones with Grandpa really really creepy...until Coconut reveals that Ginger is her GRANDMOTHER, and Grandpa's wife. Apparently she'd discovered a youth formula.
Momiji from Fruits Basket looks and acts like an elementary school kid. When Tohru finds out that he's going to go to her high school, she is shocked. He eventually goes through a growth spurt, and becomes as Bishōnen as the rest of the boys (if not more).
Souichi Tomoe, Hotaru's dad in Sailor Moon, is in his late thirties, but looks at least a decade younger, not appearing to age since the incident where Hotaru almost died. Whether it's due to Germatoid's possession's effect is unknown.
We aren't privy to her real age, but Athena of Hayate the Combat Butler appears to be the same age as the main character, but the time where she's been staying for the years before her appearance has a different standard of time. When she shows up in the current time, she looks to have aged appropriately since she's been locked out of the castle. She also acted rather like an adult back when she was supposed to be six. And her mindset is from even further back. The mindset she instilled in Hayate. And now she's back at her 6-year old appearance, while even only given the years we've seen she's at least 16.
The Blade Children of Spiral are all the same age, that is, 17... yes, even Rio. She actually uses her youthful, cute appearance to make unwitting enemies underestimate her. (And by the way, although he looks more his age, Eyes is the youngest of them all, despite being the second tallest of the main five and possibly the most intimidating.)
The main characters in Fairy Musketeers look more like 7- or 8-year olds than 14-year olds. "Juushi" could be a pun. In the title, it means "musketeer", but juushi — using different kanji — means "fourteen".
Kazan hasn't aged a day since he encountered a water demon. When the story begins he's been 6 for 10 years.
Katsuo of Yakitate!! Japan. When he first appears in the Newcomer Exam, everyone assumes he's a child because of his small stature and boyish appearance. When he makes it to the semifinals, his wife appears to cheer him on, and he reveals that he's actually 31.
Hope Stelar, Geo's mother, in Mega Man Star Force looks more like an older sister. Then again, Geo's only ten or eleven, so she can't be too old. She looks older in the game.
Mina Tepes from Dance in the Vampire Bund is, like most vampires, a definite example. The only question among fandom is how much older she is than the tween she appears to be (lowball estimates place her at her early to mid twenties, while other hints indicate she is at least on the far side of ninety).
Meiko Rokudou from Ghost Sweeper Mikami comes to mind. For most of the manga's run, her insecurity, cutesy look, and the nature of her crush on Mikami could easily make a reader think that she's anywhere from 17-19 years old (Mikami's 20-21). Then, in one of the last story arcs, we are shown Meiko's mother while she's pregnant with her. The thing is that this is the beginning of the story arc describing how Mikami's parents met. So Meiko's probably 22-24...
V.V. from Code Geass looks like a child, due to immortality, but is actually in his sixties.
Luffy, the 17-year old hero. He looks older when he is serious (and in some of the earlier arcs he was drawn with a mature look) but with his short stature, big round head and eyes, big mouth and small nose it's hard to believe he is much older than 10 years unless you know better. Doesn't really help on the age estimation that he is also a Man Child. After the Time Skip he is now 19, but looks like a 10-year old as much as ever.
Chopper looks like a baby reindeer, while he is in fact 15 (17 post Time Skip). In his semi-human form, he looks closer to his age, but no matter what form he takes, he keeps his child voice in the anime.
In SBS questions, a fan did some maths, and was amazed to discover Franky was 34. Heck with that, in Franky's flashback (8 years before Water 7), he was 26 years old! While looking just like a slight grown up from his childhood!
Sugar, an agent from Donquixote Pirates, is an interesting example. While she has the body and looks of a 10-12 year old girl, she is actually 22 years old. This is due to her eating Hobby Hobby Fruit which makes her stick at the age at which she ate the fruit, but still no less a fighter. Trebol and Lao G lampshade this.
Aokiji may look like he's in his mid-to-late twenties or early thirties, but he's actually 47/49.
Shakuyaku looks like a rather young woman, but she's actually a Retired Badass who quit piracy forty years prior to the start of the story. A flashback to twenty years before the start of the story shows that she looked absolutely identical back then, suggesting that she might simply not age at all.
Kureha looks to be in her sixties, but she's actually 139. Due to her body looking far more youthful than her Butter Face, a Running Gag with her involves people assuming her to be a young woman until they notice said Butter Face.
Martel looks like she's in her 20s but is actually old enough to be Ed and Al's mom. She has a lot of snake venom to clear her wrinkles.
Al is one year younger than his brother, but during Fullmetal Alchemist: The Conqueror of Shamballa' he looks thirteen while his brother is an adult. Justified by the fact that when he got his body back he came back as the same age he was when he tried to revive his mother.
The Affably Evil priest Rezo from Slayers is the century-old great-grandfather of one of the protagonists, Zelgadis; aside from the two barely-noticable boney lines under his eyes, he looks to be more in his late forties or early fifties. Lina lampshades this when he appears in the first Light Novel, describing him as looking "both young and old at the same time."
A non-human example would be Hellmaster Fibrizo, who is both the oldest and strongest of the world's five Demonic Lords. He disguises himself in the form of an androgynous young boy in order to decieve people.
And from the fourth and fifth seasons of the anime, Pokota the prince has the voice and the demeanor of a 10-year old. In reality, when his kingdom was struck by a plague about a decade before the series proper, his then 10-year old body was sealed away (along with the rest of the country's citizens), and his soul was placed in a stuffed animal. Realistically, he should be a bit older than Lina.
In Bakuman。, Natsumi Kato looks like a high school student or young college student, but she's actually around 30.
His guardian Misato is stated as being 29 years old, but she could pass for an in-universe 18-year old (or even younger, if not for her height and figure).
Come the third movie of Rebuild of Evangelion, Asuka and Mari are this after a fourteen year Time Skip because of a yet unexplained condition called "curse of Eva", presumably a side-effect of piloting the titular Evangelions. And then there's Kaworu, who still looks like a teen after said 14 years, due to not being human.
The epilogue of the manga reveals that Mari is even older than expected, as she was apparently knew Shinji's mother and even had an unrequited crush on her. It's unknown if this will be retained for the next Rebuild movie.
In the manga Ouja No Yuugi the magic-wielding strategists look like children or teenagers, but in fact they are adults trapped in child's body: inability to grow up is a price they had to pay for their super-powers.
Then there's Viral who will become this because of his immortality.
Ryouhei Sumi from Future GPX Cyber Formula looked like he's a little kid, but he is in fact 15 (at the beginning of the series) making him a year older than Hayato, the main character.
White from Tekkonkinkreet; by the way he looks and acts you'd think he is about 5 years old, but he's 11.
Nene from Bubblegum Crisis Tokyo 2040 is rather short and undeveloped, and tends to act like a young teenager rather than the 20-year old woman she is. It seems to be her Berserk Button, if her reaction when she flirts with her boss (His reply? “I don’t like little kids”) or when 15-year old Makkie comes onto her is any indication.
Makkie also looks like a kid who just entered his first year in middle school.
Reiji Miyabi (aka Head) from Star Driver appears to be about 15 to 17 years old. Flashbacks to the time when he fathered his now 15-year old son, Takuto Tsunashi, suggest he's at least twice that age, if not much older.
In the same vein, there is Shingo, who looked more or less the same age as Head when he befriended him in aforementioned flashbacks and just like him, he does's appear to have aged a day in the 15 years following. He's been comatose these 15 years and when he wakes up, he cryptically talks about how he "stopped growing up", but that doesn't really explain how this is possible. People have been speculating it's his first phase, but there's no official word on that.
Yuka in Kemonozume is about 30, but looks like she's in her early-to-mid twenties. She seems pleased when someone correctly guesses her age.
In the original Cutey Honey, the shape-shifting android Honey was Younger Than She Looked (supposedly 16 but was probably built more recently than that). By the time New Cutey Honey came around, she looked a little older (early- to mid-twenties, probably voluntary aging from her powers), but being over a hundred years old, she had moved to the opposite trope.
Natsu and Gajeel from Fairy Tail are hinted to be this. During the Battle of Fairy Tail arc, they get trapped with Master Makarov by a spell that only affects people over the age of 80... but the confusing part is that they're shown to have grown up normally in the past few years. It's later suggested that the spell wasn't blocking them, but the presumably ancient dragons that were sealed in their bodies, though the later revelation that they traveled 400 years from the past through Eclipse has thrown any definitive conclusion to this mystery up in the air for some.
Sun Otonashi in Rosario + Vampire looked like a middle schooler more than a year after graduating from high school.
Tooru from A Channel is 16 and in highschool, but due to her short stature she looks considerably younger.
Hanabusa Seki, Thobari's girlfriend, of Nabari no Ou. She's 40...
Yuka of Blood-C. She's apparently old enough to be a politician (which would put her somewhere in early thirties at least) but can pass as a high school student.
The title character of Acony has the body of a twelve or thirteen year old girl, but claims to be 23. Her high school aged neighbor laughs at this, saying that if she's going to lie she should go all out and say she's Really 700 Years Old, but it's true; she was involved in a superhero origin-esque botched scientific experiment that slowed her aging to a crawl. She'll look 23 by the time she's 80, but she'll die around then because her lifespan is still that of a normal human.
Even if almost all of them have a case of Really 700 Years Old, the anthropomorphic countries in Axis Powers Hetalia rarely even look their "human age". England is supposed to be physically 23, but is often drawn not looking any older than 15. Latvia's "human" age is 15, but he looks and sounds closer to 11 or 12. In-universe, this is the case for Japan; Italy thinks Japan is too young to drink, when he's actually way older than Italy.
In Arata Kangatari, Princess Kikuri looks surprisingly youthful for a 60+ year-old woman. Justified, as her powers suppress the form of her true age.
All 12 of Abe no Seimei's Shikigami in Shōnen Onmyōji are very easily hundreds, maybe even thousands of years old, but most of them appear to be in their mid-twenties or early thirties. The exceptions being Taiien and Genbu, who both look like children.
Daichi is excessively short; when Haruka first met him he was mistaken to be an elementary schooler.
Haruka herself is short among the girls, and her mother often teases her for looking child-like.
Saijou no Meii: Saijou should be in his 20's but he looks about 15 and acts even less than that. Probably to make it easier for the readers to identify with, as the author's stated goal in writing the manga was to inspire young people to study medicine.
Princess Princess: When two of the "Princesses" first met Sakamoto Akira's mother, they thought she was his sister and couldn't even figure out if she was older or younger than him. When he told them she's his mother, he even had to clarify she's not a stepmother.
Waratte! Sotomura-san: Sotomura's mother looks so young people at first think she's Sotomura's elder sister.
Medaka Box: Zenkichi's mother looks like a twelve-year-old girl but she's actually 42.
Futari Ecchi: Shoko Shiina is 24, despite looking like a little girl.
In The Face Burglar by Junji Ito, Kamei is a shapeshifting schoolgirl who involuntarily takes the appearance of anyone she spends time around. It is stated at the end of the story that she is not really a student and has been attending the school without permission, but no one knows how to get rid of her. They believe she is far older than she really appears.
The Severing Crime Edge: Iwai, due to her short stature, looks like a little girl but is actually fourteen years old.
Cassian from Count Cain's growth was stunted such that he looks twelve or so but is an adult. This being a vaguely-Victorian world, he wound up working in the circus for a while.
Kurapika from Hunter × Hunter is seventeen at the start but doesn't look much older than the twelve year olds Gon and Killua.
Biscuit appears to be a 12 year old girl but is later revealed to be a very masculine 57 year old, who uses her Nen ability to stay looking young.
Nanoka in Koi Kaze is fifteen at the start of the manga, turns sixteen a few chapters in, and is eighteen by the end. She can pass for a twelve year old which makes her relationship with someone nearly thirty ever the more squicky, even ignoring the Brother-Sister Incest.
Being a time traveler stuck in a "Groundhog Day" Loop, Homura of Puella Magi Madoka Magica has an uncertain age. Most fan estimates, based on Word of God (around one hundred loops of forty-five days) suggest her to be around twenty-six years old, next to her middle-school peers. However, she looks the same age as the rest of the cast - in fact, she's actually a Pettanko and the second shortest girl. Explanations, from side-effects of her powers to actual immortality, abound in the fanbase.
AKIRA: Kyoko, Takashi and Masaru were often mistaken for children; even a police officer does this when he saw Takashi. However, the trio is actually in their late 30s, with Kyoko being oldest in her early 40s like The Colonel. This is due in part of taking medicine to keep their powers in check.
Yukiya from Okane Ga Nai is hit seriously with uke stereotypes. He's an eighteen year old college student but looks more like a twelve year old boy, or a eighteen year old woman.
The Demons in Kyo Kara Maoh!! age at a slower rate than humans, and live longer as well making them this. A good example is the main hero's accidental fiance Wolfrem, who is physically a 16 year old, but is chronologically 82.
Toriko was surprised to find out when they were at an island known for its alcohol that his partner Komatsu (who looks like a preteen) is the same age he is — 25.
Hinata Hakamada from Ro-Kyu-Bu! is a sixth grader but looks and acts like she's around seven. Subverted as she does have noticeable breasts. Humorously enough, her "little" sister Kagetsu is Younger Than They Look.
InuYasha: this is common for youkai characters due to their supernatural heritage.
Inuyasha is set up to appear, and behave, as though he's the same age as the fifteen-year old Kagome. However, he is half-youkai and his appearance has not changed in 50 years. Flashbacks of his mother show her dressed in Heian-era aristocratic clothing, which would imply that Inuyasha could be as much as 500 years old. The third anime film declares that he is 200 years old, although there's no age given in canon.
Rumiko Takahashi has stated that Sesshoumaru is the youkai equivalent of a nineteen year old (although the anime ages him up to look like he's in his twenties). He is Inuyasha's older brother, so he's definitely more than 50 years old. Given that the third anime film makes Inuyasha 200 years old, the anime team regard Sesshoumaru as being even older than that.
Sesshoumaru's mother looks barely any older than Sesshoumaru himself. There is no indication how old she is in the manga, but she's been around for more than fifty years.
Shishinki is an old enemy of Inuyasha's youkai father, but looks barely older than Sesshoumaru. Inuyasha's father died when Inuyasha was a young child (Inuyasha tells Sesshoumaru he doesn't have many memories of their father as a result), and Inuyasha is over fifty years old. When Jaken hears that Shinshinki was the father's enemy, he's shocked by how youthful appears to be.
Soul Reapers have vastly expanded lifespans and therefore their chronological ages do not match their biological ages. For example, Yamamoto looks like an elderly grandfather but has been around for over 2,000 years whereas Hitsugaya looks like he's around 13 years old but has been around for at least 40 years. While Rukia looks like she's in the same age-group as Ichigo, she once told his fifteen-year old self that she had lived ten times his lifespan.
Hollows don't age at all. As a result, there is no way to guess how old they might be. Lilinette lampshades this when Ukitake refuses to fight her on the grounds that he cannot - will not - fight children. He knows her chronological age has nothing to do with her biological appearance, but he will not change his stance.
The age of a Quincy cannot be judged by their appearance. Although they are regarded as humans and are born in the World of the Living, their souls are classified as being different to human souls, and several Quincies are confirmed to be around a thousand years old. Most of the Quincies left the World of the Living to hide in the Soul Society, so it's ambiguous as to whether they're now living humans with unnatural lifespans, or have become more akin to the Soul Reapers and Hollows (spirits with unnatural lifespans). Yhwach himself was at least 200 years old before he first travelled to the Soul Society despite looking like he was only in his thirties.
In the filler Bount arc, Ran'Tao looks like she's around 30-year-old but is very old even by Soul Reaper standards. Through self-experimentation, she greatly slowed her aging at the price of greatly reduced stamina and a need to sleep roughly 2/3 of each day.
Suzu from Seitokai Yakuindomo looks like an elementary schooler, but is actually in high school. She's a little touchy about the subject.
Nanami in Yandere Kanojo is another high schooler who looks like she's still in elementary. This causes problems when she falls for main character Manabu, who thinks she is the age she looks, and treats her as having a Precocious Crush on him.
Also of note is the Principal's wife, who looks as if she's still at school, but was actually classmates with her husband. The other characters even lampshade just how weird it is.
If Her Flag Breaks: School elder Tsumugi Ryuukishibara looks like a young child, complete with bonnet; in truth, she was once a fully-grown woman, but shrank to her current height as she got older. No-one knows how, and Tsumugi herself seems to think it's perfectly natural. Rumor has it she's been around since the school's founding.
Chiharu the fox demon from Fuuun Ishin Dai Shogun she looks like a 13 or 14 year old girl but she's 250 years old.
Aladdin in Magi – Labyrinth of Magic starts off the series as a ten year old and biologically he is. Chronologically he is a lot older due to being suspended for years as a fetus.
Jim, Adam, Turkes and Lint in Mother Keeper are all this. Lint and Turkes are both cyborgs so it's understandable why they haven't aged, however Jim would presumably be about 60-70 if she made Lint yet she doesn't look older than her 30s. Turkes makes a mention to the fact he's known Adam 20 years and he hasn't aged at all either.
Yuzuru from Koe no Katachi is a middle schooler but several times gets mistaken for an elementary schooler.
Mikako from Himegoto - Juukyuusai no Seifuku is eighteen but tells the men she has sex with that she's a fifteen year old, and dresses in a Sailor Fuku when doing so. She started prostituting when she was fifteen so when she tells men she began "three years ago" it is not technically a lie.
Touka Kirishima mentions that Kaneki Ken still has a "kid's face", and easily passes for a highschooler. He's also mistaken for a high school student by one of the Ghoul Restaurant patrons.
Juuzou Suzuya looks like an androgynous youth (or a woman), with people actually expressing surprise when they realize he's an adult. It's justified in his case, since he was castrated to prevent puberty, and so will never really look like a grown man. He's 19 in the original series, and 22 in the sequel but still looks like a teenager.
Chie Hori is incredibly short and has a round face, causing people to mistake her for a Middle School student. In Tokyo Ghoul:Re, she actually has to produce her driver's license to verify her claim that she's 24.
Link in one of The Legend of Zelda manga based off the original game is older than the teenage Zelda but looks like a little kid because he's half-elf, and elves age slower than humans. He is in fact her half-brother.
Maria claims to be an adult in The Testament of Sister New Devil, and produces a fake id to show to cops in case she gets stopped on the street during school hours. However, Basara wonders how effective it'll be as she looks younger than him.
Chiyo Sakura from Monthly Girls' Nozaki-kun gets this, as she's very short. Seo even accuses Nozaki of being a Lolicon after she notices him hanging around Sakura.
Akatsuki of Log Horizon looks like a middle schooler but is actually in college.
The Ryoto family from Wild Life turns this Up to Eleven. Tsukasa Ryoto is probably in his mid-40s, but still look like a 20 year old. His father and grandmother looks the same age, even though the latter is 100 years old.
Zeno is seventeen, but looks more like he's 14 or 15. And he certainly doesn't look over 2000.
There's also Kija, who's 20 but looks (and acts) younger than the eighteen-year-old Hak. This fact is lampshaded in chapter 114 by Yoon.
Later chapters introduced Princess Tao, a princess from Kouka's neighbouring kingdom of Xing, who looks to be no older than 13 or 14. She's 19 in reality, 3 years older and far tinier than the manga's protagonist, Yona, who's fairly petite herself.
Kirin from Gourmet Girl Graffiti is often mistaken as an elementary school student. Due to a lack of Comic-Book Time, she started as an eighth-grader and is currently (as of April 2015) a tenth-grader.
Charlotte B. Lord, the school principal of Midgar in Unlimited Fafnir, looks like a ten year old girl. Yuu, at sixteen, is much taller than she is as well.
Akira Toriyama has stated in one interview that Arale the android from Dr. Slump is meant to be 13; however she looks and acts like a 7 or 8 year old.
Nico Yazawa in Love Live! is sometimes mistaken for a first-year student, due to her pigtails, childish antics, and less developed body. However, she's actually a third-year student like Nozomi and Ayase, who are taller than her.
Harpies in Daily Life with Monster Girl are generally like this, though it's justified by their bodies needing to be small and light for the sake of flight. The harpy among the cast, Papi, looks like a young teenager, prompting everyone to regard her as a child and, in the case of Miia and Centorea, means she isn't a potential rival for Kimihito. This changes upon learning that, despite her small body and childlike nature, she's roughly the same age as them.
Papi's, Miia's, and Centorea's mothers all look very young, reaching Mother Looks Like a Sister territory. Papi's mother is justified since shes a harpy, Miia and Centorea's just looking that young, and Mero's mom while not as much as the others is quite young looking. This could be explained by how most mythological creatures have much longer lifespans than humans.
Rino looks and acts like she's 5 despite being 25. Her husband Nozomu even mistook her for a Child Prodigy when they first met in college.
Mayotama says that people often think he's in junior high when he's actually a college student. Of course, he's 1.55m tall, some 19cm shorter than the average Japanese male (but around the female average).
Marube from Yuureitou is a more realistic example than most. He's in his forties but can pass for someone ten years younger. He works out. This is foreshadowing for his obsession with staying young and beautiful. He wants to become a woman (though it's superficial instead of being trans) in order to be so. He looks quite attractive dressed up despite his age.
Minoru from Occult Academy is apparently old enough to smoke and be a high school teacher yet doesn't look much older than his students.
Sgt. Frog: Pururu. One episode even has Keroro catching a glimpse of her face sans makeup in a fantastic Unreveal.
Space Policewoman Poyon is implied to have capured rogue aliens on Earth since feudal times.
Aoba and Nene of New Game!, both 18 or 19 years old, have been mistaken as middle schoolers in-universe.
The main girls in Petite Princess Yucie are seventeen years old but appear no older than ten thanks to a spell cast when they were babies.
Kenjiro Minami is 17, but he looks younger than Yuri Plisetsky (who's the youngest competing skater at 15), being shorter and very baby-faced.
Minako Okukawa is one of the few named characters whose official age hasn't been revealed, but Word of God has stated that she's older than Yuri's mother Hiroko (who's 49 and calls Minako "senpai"), which would put her in her fifties. Despite that, she looks even younger than Yuri's sister Mari, who's 30.
Dragons in Miss Kobayashi's Dragon Maid are all Time Abysses, but the only indications Iruru is any older than Kanna are her Gag Boobs. This is due to Iruru having poorer shapeshifting abilities than the other dragons, resulting in her unbalanced appearance. She's actually around the same age as Tohru.