Some characters are subject to a strange fluctuating sense of maturity depending on plot necessity, mostly because animated characters can have abstract designs and lack obvious visual cues. We're only left with the personality of the character, which tends to suggest their age. Dawson Casting, leading to a blurring of the line between adults-playing-adults and adults-playing-teenagers, is responsible for several live-action examples.
A TV character may be implied to be some sort of adult compared to other characters who are clearly "kids", or vice versa, but this is largely an illusion in case the writers still want them in plots a younger audience can more easily relate to. The only nominally-aged character is usually the lead, and this is only useful to make his same-aged audience feel better, since he will probably not be allowed to get older.
Occasionally, characters are given specific ages along with more obscure and extensive biographies, but are ultimately ruled by audience appeal, Cast Speciation, and personalities that fit into the mean age of the audience. Information that runs counter to that turns into the Improbable Age issue, and may be tweaked in other places.
Compare Mistaken Age.
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Anime and Manga
Mireille of .hack//Legend of the Twilight, who can be very knowledgeable at times, sometimes lord it over others, and sometimes just be cute and vulnerable. She actually turns out to be the four-year-old daughter of an identical character from the first game, using her mother's file, and one of the other girls in Shugo's harem is her home tutor, explaining some of it.
Lupin III: The characters are certainly adults, but beyond that it's anyone's guess. The fact that they have been having adventures since The Sixties can be ignored since the series runs on Comicbook Time. However, Lupin himself is canonically the grandson of Arsčne Lupin, who first appeared over a century ago, already an adult. At this point, the timeline would make a lot more sense if it were called Lupin the Fourth or even Lupin the Fifth.
Baccano! makes it rather difficult to determine Ennis's physical or chronological age, as her origin is somewhatcomplicated and not too many details are given on her timeline in either the anime or Light Novels beyond three tidbits: she was around before the commercialization of radio (with the given setting, means she can be anywhere from eight to two hundred and nineteen), she's physically the same age as her "mother" was when she unwillingly "donated" some of her cells (nothing is given about this girl/woman other than that she was female, so that gets us nowhere), and that she identifies Firo on sight as "around the same age as her, or a little younger" (which, given that Firo's an eighteen-year-old regularly mistaken to be as young as fourteen or fifteen, also gets us nowhere).
Played with in Haré+Guu. When attending The City's school, Haré immediately notices all the kids are separated by grades. His own school is a literal one-teacher and his homeroom consists of every non-adult in the series, including teenagers. note Which incidentally is Truth in Television for extremely low population density areas; a single teacher is able to give individualized assignments for each student based on their age, when there's only a handful of them to begin with.
Kobato. She certainly acts a lot younger than a teenager, but from her design and the way other characters treat her she seems to be at least twelve and probably not old enough to be in high school.
Applied to Luna the cat in Pretty Guardian Sailor Moon, who despite sounding identical to the version in Sailor Moon (who was a Vague Age full grown cat until she briefly became a human(oid)), was given the body of a young girl as a occasional human form. She was described by the mangaka as being a year younger than the lead character and became a teenaged girl when human, but also has been alive since the Silver Millennium.
Likewise her younger brother's age can fluctuate from between two and four years younger than her depending on whether you're watching the anime, the live-action series, or reading the manga.
Sunrise has been rather opaque about the ages of the heroes in Tiger & Bunny. So far, they've only given out two and a half: Barnaby's 24, Kaede's 9, and Kotetsu's kinda sorta probably somewhere between 33 and 39. Maybe.
Coby of One Piece appears to be child much younger than Luffy (who is 17 at the beginning of the story), but enlists in the marines as soon as he's saved and then apparently goes through puberty and has a growth spurt by the next time we see him.
Shakky is an even better example. She mentions that she was chased by Garp and stopped pirating 40 years ago, which would probably make her more than 60 (at least not a day under 55). But she looks like she is in her late twenties or early thirties, and the characters have questioned her age several times. So she could be anywhere from 60 to more than 100 since it would be unrealistic for an older woman to look like a 30-year-old no matter what.
Satoko and Rika from Higurashi no Naku Koro ni. They're noticeably younger than the protagonists but how much younger is never quite stated. Satoko seems to have entered puberty but Rika hasn't yet. The closest we get to an age confirmation is in the original sound novels, and even then all we know is that Satoko is anywhere from 9 to 13 years old.
Amae Koromo from Saki. Though she appears very child-like, she is a second-year student as opposed to the majority of Kiyosumi High's players like Saki, Nodoka, and Yuuki who are first-years, and it's been hinted that she's older than Nodoka.
Satsuki Hyoudou from Maid-Sama!. Despite looking like a teenager, she is actually 30 years old.
Mikuru Asahina from Haruhi Suzumiya. Although she looks like a moe teenager, when Kyon asked her how old she really is after she revealed her true origin and purpose, she merely responded, "That's classified."
Levi (and a few others) from Attack on Titan. Levi is noticeably smaller than many character, including the 15 year old protagonists, and looks younger than a few of the trainees, but in reality he has to be anywhere from three to ten years older than them.
InuYasha: Kikyo is accepted to be in her mid-to-late teens, but her exact age is uncertain. The InuYasha Profiles book states that she was eighteen when she originally died, whereas Rumiko Takahashi said she was "around seventeen" when it happened.
Likewise, the exact ages of all youkai and hanyou characters is left deliberately ambiguous.
Astérix: Asterix himself is described as being "older than he looks." Between the art style used for all the characters, and the features of Asterix himself, he looks somewhere between 20 and 60. What that actually makes him is anyone's guess. Lampshaded in The Cauldron, where he's working for a wrestling promoter who actually describes him in his 'roll-up' pitch as being 'of indeterminate age'. (One short story strip of dubious canoninity (due to being outside of an official volume and containing some inconsistencies with other stories) states Asterix and Obelix are 35 "when their adventures begin".)
Donald Duck's nephews, Huey, Dewey, and Louie in Carl Barks' comics. In one story they are in kindergarten; in another they are shown driving a car! (The "kindergarten" they are attending is a rather unusual one, however; in the background of the first panel, there appears to be an integral equation written on the blackboard.)
Also Donald Duck himself. It's obvious that he is not really young or old, but it's not clear (and it depends on the writer) if he's still somewhat young like in his late twenties or he is middle-aged.
Franklin Richards, the child of Fantastic Four members Mr. Fantastic and the Invisible Woman, is somewhere between four and ten. Whether his reality warp power cause the age vagueness or if the age vagueness causes the reality warping is up for debate.
Gaston Lagaffe: Lagaffe is old enough to smoke and have a car, a job, and his own place, but young enough to constantly play around at work and bawl his eyes out over very little.
Spirou and Fantasio: Spirou spent the 1950s stuck in a limbo where he looked like a teenager (being one head shorter than his adult friend Fantasio), but drove a car and had his own apartment. He finally became unambiguously adult shortly before or after André Franquin left the series.
Jimmy Olsen, Superman's Pal, would go back and forth between being old enough to drive, have a pilot's license and drink, and still being in high school, touching on every stage of life in between at least once.
Lampshaded in one issue where a childhood-obsessed, adult hatingversion of the Toyman tells his story to Jimmy Olsen alone because he doesn't trust anyone over the age of eighteen. Jimmy kicks him over and shouts that he's Twenty-Three years old exactly.
Tintin: It's never made exactly clear how old Tintin is. He is obviously young, but has no problems going round the world and getting into adventures without any mention of parents or the like. In response to this, Herge was actually asked to create a character with a family, mum, dad, little sister, pet, etc. in order to have a more "realistic" series of adventures. The result was Jo, Zette and Jocko, a fun, but less successful series. As Herge pointed out, it was much harder to keep track of all those characters. At least good old Tintin was unencumbered. Not everyone gets to be an orphan.
Scott Pilgrimlampshaded this all the time with Ramona Flowers. While every other character would have their age blatantly stated every time they were introduced (especially Knives Chau, Age 17), Ramona's introduction was always followed by "Age Unknown." At one point, Scott lampshades the lampshading when Ramona gets mad at him for not knowing how old she is. He defends himself with, "But your age is unknown!"
Granted, The Smurfs are defined as 100 years old, but what makes them an example of the trope is the fact that it's unclear which human age equals that; most of them have specific jobs inside the village, yet they frequently play ball and act immature. Maybe one thing or the other may be the consequence of living in a small village led by their father (Papa Smurf is no mere name — he raised the 98 Smurfs). They were more clearly defined as adults when three of them were age-reversed to Smurflings, which made the adult Smurfs behave somewhat more mature and proved that we were better with the vague age.
This gets even more complicated with the appearance of Gutsy Smurf with his sideburns in the live-action movie, since most of his fellow Smurfs don't have facial hair.
Unlike other versions of the series, Sonic the Comic never stated the age of anyone. Their ages were either left ambiguous or never stated at all. The series goes by its own canon, so we can't assume that most characters fit the game age either. Tails is stuck somewhere between eight years of age and fourteen, while Amy never really did seem to be as young as she was in Sonic CD (which was eight).
Doctor Strange started out somewhere in his fifties, though the standard depiction is of a man somewhere in his mid-forties. Even so, the only real indication of his greater age are some lines on his face and the Skunk Stripes of greying hair at his temples.
The pygmies in Pocket God may appear to be small children given their size, but most of them don't act like it. They don't even know their own ages or how long they've been around; probably as a side effect of their Resurrective Immortality.
Thanks to her Healing Factor being forcibly activated at the age of seven, X-23's age becomes much less clear. She looks about 17, but because her aging process is slowed by her mutation, she can easily be years older than this. However a closer look at the dates given in her various solo appearances does confirm an age of 17 by the time she joins the X-books proper: She escaped the Facility at 13, was 14-15 while staying with her cousin and aunt (which can be verified because Megan was still in high school, and is known to be a couple years older than Laura), and 15-16 during NYX.
Most of the main cast of The Bunker, In some episodes the gang is shown driving, In some episodes ACF appears in, she looks like a Powerpuff Girl.
The Agents in Akatsuki Kitten Phoenix Corporation Overhaul are implied to be somewhere between mature teenagers and in their twenties. This is muddled by the fact that they don't live in a linear timeline, and some of them simply look older in a relative manner to the others. For instance, Agent Diamond is less than five feet tall (despite her impressive bosom), and is a bit of a Psychopathic Manchild, while her boyfriend, Agent Chimera, is six foot three and generally quite mature. All but one of the Agents are stated to have been born within two years of each other.
Averted in the Empath: The Luckiest Smurf series, as Empath and his fellow Smurfs are depicted as being in their early adult stage around 150 years of age, with Homnibus commenting on how young Empath looks for his age.
Jack Skellington from The Nightmare Before Christmas. In personality and voice, he comes across as being in his late twenties or early thirties, but he's an undead skeleton man, so this has no connection whatsoever to how long he's been around, and aside from being adult-sized it's impossible to gauge his age from his looks. He's probably older than thirtyish, though it wouldn't be impossible for him to be younger.
The video game characters in Wreck-It Ralph. While all game characters are technically immortal unless they die along with their game or outside of it, Ralph looks to be in his 30s while Felix and Calhoun appear more in their late 20s. Meanwhile, Vanellope and all her racing rivals are supposed to be 9 - 10 years old.
The games they come from (and by extension, the characters themselves) have different ages. Ralph and Felix have been around for 30 years. Vanellope's game has been around for at least 15 years and Calhoun's game was only installed a week before the events of the movie, making her the youngest character chronologically.
Dorothy in The Wizard of Oz is supposedly meant to be under ten years old, but Judy Garland couldn't pass for a little kid even with Suppressed Mammaries. Most people who watch the movie think Dorothy's meant to be between twelve and fifteen.
Apparently done deliberately in casting Malcolm McDowell as Alex in A Clockwork Orange. The character is revealed to be fifteen in the book, while McDowell appears to be somewhere in the rage of late teens to early twenties.
Vardaman from William Faulkner's As I Lay Dying has no set age, and it is often debated as to whether he is young or just retarded (or both).
In A Series of Unfortunate Events, Sunny is exceptionally clever for a baby, and her physical development occurs in a deliberately uneven, contradictory way; for example, even after she learns to walk, she is small enough to sleep in a casserole dish. Her exact age is never stated, and flashbacks to "before Sunny" carefully avoid giving the other siblings' ages at the time, forestalling calculation. Averted in book thirteen, which stated Sunny was about three, which would mean she would be about six months at the most in book one, meaning about two and a half years have passed since.
Everyone in The Moomins, though some are more vague than others, and especially Snufkin.
Ascanius, Aeneas's son in the Aeneid, is at varying points in the poem young enough to curl up on Dido's lap at dinner (or at least, no one thinks it's weird when Cupid impersonates him to do so), and old enough to fight in the war. He appears to be somewhere between four and fourteen, never specified.
Diana Wynne Jones deliberately kept the ages of her child-characters vague unless there was a plot reason not to, to broaden their appeal to child readers. At a certain stage it can be off-putting to find out that the hero of the book you're reading is younger than you are (if you're eleven, nine seems pretty young). In Hexwood, some characters' Vague Age is a plot point in itself.
This was done in Animorphs to make the characters more relatable to a wider audience (in universe done to protect their identities). This was changed in the last book when it was revealed that they were aged 13 in the first book and 16 in the last.
In the Book of Genesis, the story of Isaac's near-sacrifice has him speaking to his father Abraham the way a young child would, asking him, on the way up the mountain, where the sheep was for their burnt offering, and Abraham replying evasively, "God will see to the sheep for his burnt offering, my son," as though to shield him from the horrible truth until the last moment. However, based on the internal chronology of the book, Isaac would've been thirty-seven years old at the time. Thus, the Jewish sages of late antiquity exploited this ambiguity, portraying Isaac as old enough to give informed consent to his own sacrifice.
Chichi deliberately invokes this in Akata Witch. She refuses to tell anyone how old she is, except when she's teasing an older man. After she gets him excited, she tells him she's twelve (or whatever age she feels like being), laughs at his horror, and runs off.
Ozma's age in The Wonderful Wizard of Oz is never concrete. Some illustrations have her as an adult, some as a child. Typically she's depicted as being Dorothy's age though.
In the Rainbow Magic series, it's never stated how old Rachel and Kirsty are supposed to be. They could be anywhere from very young to middle school aged.
Professor Cole in Murderess, who can transform into a teenage boy and back.
Live Action TV
30 Rock: Kenneth appears a bit too old to be in the age bracket likely to work as a network intern, but there's also quite a bit of evidence that he might actually be older than he appears. No definitive answer has ever been given. Seriously, just check out the Older than They Look section on the series' main page. If he's to be believed, Kenneth has been working for NBC since 1936.
Blues Clues: How old are the live action cast supposed to be? Apparently Steve is an adult, maybe a late teenager, but what about his brother?
Community: Abed is somewhere in his twenties, but where is anyone's guess.
Merlin: Has never given any definitive ages for its cast, though Word of God has it that of the four leads, Morgana is the oldest, followed by Arthur, with Merlin and Guinevere clocking in as late teenagers when the show begins. When taking into account the Time Skips that occur between seasons, nearly ten years have passed in-show, meaning that the cast (all in their mid-twenties when filming began) went from Dawson Casting to Playing Gertrude.
The Muppets: Most of them, since they tend to have abstract appearances anyway. For Sesame Street characters, this is done for psychological appeal; the regulars are supposed to emulate different stages of childhood development, and each character's 'official' age is just a reflection of that.
Revolution: The characters have not been given ages for the most part. However, Danny Matheson is supposedly 19 years. This is reasonably close the character's actor Graham Rogers's age, which is 17 years old. Captain Riley is apparently the youngest captain the Monroe Republic has. The actor playing the character Omid Abtahi is 34 years old.
In the first season, Zack and Cody are presumably the same age as their actors: 12 or 13, while Maddie and London are in their late teens. However, in later seasons (and in the spinoff), they are clearly older teens, while Maddie and London are the same age: In the spinoff the twins and London go to school together.
When Zach and Cody graduate in the spinoff, their actors are about 18/19 years old. London graduates at the same time, despite originally being a few grades ahead of them, because she kept flunking grades.
The Vampire Diaries: All of the Originals ages are unknown. Rebekah and Kol are teenagers, where in their teens they fall is anyone's guess and Klaus, Elijah, and Finn are total mystery. It's also unknown at what age Katherine was turned.
In the seasons of Power Rangers where the characters are not high school students, the Rangers' ages tend to be left vague at best. They are often provided in promotional materials (for some characters), though even these are notoriously unreliable: Bridge, the only character in Power Rangers S.P.D. with a definitive age, was listed as 17 on the Jetix website but is explicitly 21 in the show proper.
FarScape, for much of its run, which is odd considering how often the show mentions time passing. In the first episode, D'Argo's age is given as 30 cycles (young for a Luxan, which means he married very young - a plot point). Everyone else's age stays mostly vague. Chiana is often stated to be "a kid" compared to the others, and acts as though she's in her late teens/early twenties. John's age can eventually be inferred as about 30 when he joins Moya, since he's old enough to drive in 1986 and was presumably born in 1969 or 1970.
Couette from Noob. She's short and acts like a sterotypical teenage girl, but is also the Distaff Counterpart of the franchise Man Child undergoing Divergent Character Evolution. Her actress in the webseries is a short adult, but she also hints to still be going to class in some of her dialogue. The comic shows the other contestants at her beauty pageant to be much taller than her and that she lives in her own house, but has her drawn with the same standards as the Man Child in question, making look her like the youngest of all regular players.
MapleStory allows your character to marry (For NX, of course,) even if they look like they're not even close to maturity. Some of the quest givers give off the impression of being young (i.e. a student attending a magic school), but appear to be just as tall as your character and don't have a clear age either. In one quest line, you begin by playing hide and seek with children. A few levels later, you're fighting in a revolution.
In Little Nemo, Flip's age is perennial a mystery, referred to at different points as a child and 23, and the same height as Nemo with a receding hairline and cigar.
Musicals in general tend to have a case of this. Unless it is plot relevant, a character's age will probably be vague enough to allow actors spanning a decade to play them. Possibly more.
Hamlet. He's a university student, he was passed over as heir when his father died, and the play is more or less his Coming of Age Story, all of which would seem to put him in his late teens. But the scene with the gravediggers gives his age as thirty (or thereabouts). The historical explanation is that Hamlet was probably originally intended to be a teenager but that Shakespeare changed his age so his usual star Richard Burbage could play the role.
Early in his series, Link's age was indeterminate, and his sprite had too little detail to tell. In-manual art drew him like Peter Pan with more hair, making it more unclear how old he actually was. The exception is in Zelda II: The Adventure of Link, where he is explicitly 16 years old. Most of the later console games give him a more explicit age, but the handheld games are still vague. It wasn't until much later when Link in The Legend of Zelda I was confirmed to be 12 years old from the same source that stated that Ocarina of Time Link was 9 years old as a child. Link in The Legend of Zelda: A Link to the Past may be implied to be a kid, not only by the AV Famicom commercial (which oddly promotes the cartridge port of The Legend of Zelda I), but also by the GBA port of the game, where he is given young N64 Link's voice.
His age isn't stated, but is generally assumed to be a between 9 and 16 in The Legend of Zelda: Majora's Mask, as he is younger than the 16-year-old model but it is stated to have been a while since the events of Ocarina of Time, at the end of which he stayed 9.
Zelda herself is an oddity. Her age is never given but she rarely, if ever, seems too different in age than Link. Twilight Princess Zelda is a source of common debate: Is she older than the roughly 16 year old Link or is it simply her stoic and mature personality that gives the impression?
Princess Peach. In the games (especially in her 3D appearances) she has a decidedly mature physique and is even taller than Luigi; however, in the comics she acts more like a tomboyish tween, and her relationship with Mario seems to be one of childish infatuation. Also, in Paper Mario: The Thousand-Year Door, the Shadow Queen refers to her as a "child". We do get one age confirmation on the DVD extras of the Super Mario Bros. 3 cartoon where she is supposed to be 17 years old. But the cartoon isn't really canon, so the question still remains.
This also applies to Mario, Luigi and Bowser. In Mario & Luigi: Partners in Time, we see the three, plus Peach, as infants; yet in their adult years, Peach hasn't lost her youthful look. The Mario brothers are implied to still be in their twenties, and they're just a bit older than her.
Mario was confirmed 26 in his Japanese trophy description for Super Smash Bros. Melee. Luigi is his twin brother (but born after him) so he is also 26. Peach and Bowser still remain a mystery, but since Peach couldn't even walk as an infant in Partners in Time and Bowser could talk as an infant, which the baby Mario Bros. can and can't do, respectively, Peach is implied to be a year or so younger (around 25) and Bowser a year or so older (around 27).
Yoshi. The official voice was introduced in Yoshi's Story for a group of Yoshi infants, yet in later games, the same voice is used for its grown form. In Paper Mario: The Thousand-Year Door, it gets even weirder as the Yoshi there apparently hatches out of the egg with vague knowledge of the world ("Is that one of those 'kiss' things? I've go to get me one of those!") and no name, but talks like a teenage punk and generally acts like an adult.
Yoshi's (the main one's) infant form also appeared as a cameo in the ending of Yoshi's Island DS, making it even more confusing.
Kamek. He's presumably pretty old given how he was an adult back in the Yoshi's Island series (and is still around now), but how old? Who knows, he hasn't even changed design in the 20 odd year time skip. Only thing that ever changed for him was the colour of his robe and wizard hat (and by one shade of a blue at that).
Rosalina. She looks in her late 30s or 40s, but visits the Mushroom Kingdom every 100 years. She is implied to be immortal, but that doesn't explain why she physically appears younger in the optional flashback story.
The questionable art skills of Zun, Touhou's creator have made the intended age of most characters unfathomable from their official portraits. Since Word of God stays extremely vague on the subject, this has left the fandom free rein on their age. Even more than the others, Orin and Patchouli (who wears baggy clothes) have fallen victims to this trope, alternating between cute girls and fully grown women, Depending on the Artist. Even limiting it to the humans, whose ages presumably matter more, it's extremely unclear how old Marisa and Reimu are supposed to be, with the range often being given as things like 10-20. And even if we knew their precise age at one point, it's unclear if they're aging or not. If each change of the seasons indicates a year passing in-story, they'd be somewhere in their 20s by now. Which isn't entirely implausible, but it's just hard to tell.
Amareus of Yggdra Union is one of only a handful of characters in the game whose age is not explicitly stated somewhere or other. Instead, the official website gives us a ballpark-estimate quote: She's "2? years old".
The three main characters of the Professor Layton series all experience this in the original trilogy. Layton himself, judging by what was learned of him in Unwound Future, is in his mid- to late-thirties. His self-styled apprentice Luke insists that he's too old for most toys, but still wears short pants (the games are set in Anachronism Stew England) and is clearly shorter and younger than Flora. The only clue given for Flora's age is that the events of the first game happened after she was "out of childhood," but she's still young enough to require a guardian; most players estimate her to be between fourteen and seventeen.
Ages for Layton and Luke, at least, are mentioned in the Japanese version. Layton is 37 in the original trilogy, Luke 13. Still no age for Flora, though.
The Last Specter finally states Layton and Luke's ages (but not Flora's, which makes sense as this game takes place before they meet her). Layton became Gressenheller University's youngest professor at the age of 27, which confirms his being 37 in the original trilogy. Luke is 7 in Last Specter, making him 10 - not 13 - at the time of the original three games.
Basically everyone in Pokémon, especially the protagonists. The only protagonists with a confirmed age is Red, who's eleven. Leaf and Blue are presumed to be eleven too. Wes is stated to be around seventeen - eighteen, and a spinoff girl is said to be just a bit older than him.
The Pokémon Black and White protagonists are stated to be older than previous heroes of the main series, but it's unknown by how many years.
Many people assume the player characters are around 10 thanks in part to the anime making Ash 10, but again, the only characters in the games with outright stated ages are a year older than that. Also, Michael is identified as "an unknown teen" in Gale Of Darkness, so he could be at least 13, yet he looks about the same as the characters from the main games (not to mention he's about a head shorter).
The Pokémon X and Y protags muddy these waters even further; thanks to the Virtual Paper Doll feature of the games, their clothing and hairstyles can make them look older or younger. On one extreme, "cute" outfits can make them look like the ten-year-olds everyone associates with Ash and Misty (Serena even getting some Misty-like outfit options), while on the other end of the scale, outfits from Snowbelle or Lumiose can make them look like early college students. Even more confusing is that your character has to bend down to talk to young children, but not all children. The very best we get is that a character who is roughly the protagonists' height is identified as being sixteen.
Terra, Aqua and Ven of Kingdom Hearts: Birth by Sleep are never given official ages, unlike Sora, Riku and Kairi. Though if you take some things into consideration, such as Ventus being physically identical to Roxas, nobody of fifteen year old Sora, and Word of God telling us that Xemnas is about thirty, it's not that much of a stretch to assume that Ven is about fifteen and Terra is about nineteen. All we get for Aqua is that she's younger than Terra and older than Ven.
Maxwell from Scribblenauts is never given an age, though Scribblenauts Unlimited establishes he's still a kid.
Your fellow villagers in Animal Crossing are... mystifying, to say the least. They live on their own, so they're adults, right? But some of them mention still having allowance, and some of their birthday messages imply they're not yet adults. But there are no parents anywhere (and, in fact, the kangaroo villagers have children of their own). The "Grumpy" villagers act older than the others, with some of their dialogue implying they're probably in their early 30's (they mention not knowing what's popular with kids, but aren't implied to be too much older than the others), but with the rest of the clan, it's anybody's guess.
The protagonists are equally vague. You look like a child but are implied to be an adult. The newest game has changed from Super-Deformed to a more lanky style, with mixed results.
Nobody knows how old the villainous Doctor Eggman from Sonic the Hedgehog is supposed to be. The only clue about his age is that he was alive when Professor Gerald Robotnik perished 50 years ago, implying he must be at least that old.
Characters from Parappa The Rapper. For an example, Parappa himself is old enough to drive a car and join the military but not old enough to work.
Kirby was described in the Manual of Kirby's Dreamland to be a "little boy" but gave no age, and his age in games since then hasn't really been touched upon.
Some of the characters in the WarioWare series fall into this. Ashley's the most obvious example (her 'official' age is supposedly around 15 according to the US website, but might be different in Japan), and her design is so vague that it's kind of hard to tell what age the character is outside of 'looks fairly young'. Same deal with 18 Volt, who looks like anything between a high school student and adult, who's somehow still in elementary school.
The characters in What the Fu are only defined as somewhere in their twenties, mostly because Zac can't be bothered with details.
In City of Reality, none of the main characters have defined ages — this is deliberate on the part of the author so as not to trip himself up with potential romantic subplots. The cartoonish art style doesn't help matters. However, it is possible to extrapolate from their statements about graduating high school and the assumptions people on other worlds make about their youth, and they got a visual (though again not textual) age-up between chapters 5 and 6.
Cucumber and his sister Almond of Cucumber Quest are, in the author's own words, "A young kid and a slightly younger kid." We do know that Cucumber isn't 9, because that's how old his father thinks he is.
Sinfest is pretty weird with this. Word Of God is that Slick is somewhere between 14 and his early twenties, and the other characters rarely (if ever) get their ages touched upon. It's especially weird considering that most of the main male characters look considerably younger than the women (who all look somewhere from their late teens to twenties). It's not helped by the fact that just about every character lives on their own.
The author of Schlock Mercenary, has confessed that he has no idea how old General Tagon is, guessing that he's somewhere between 18 and 40 years older than his 49 year-old son.
Happy Tree Friends: According to the creators, the Tree Friends are whatever age is necessary for the cartoon to work, aside from a handful of characters who are always portrayed as adults (though rarely competent).
Homestar Runner has only two or three confirmed "adult" regulars; the others can be, as the plot requires, young enough to be going to preschool or old enough to be drinking Cold Ones. Even the adult regulars aren't too old to play spaceman with tin foil outfits and cardboard spaceships. Don't even ask about Homsar.
The flash cartoon series Retarded Animal Babies has mocked this a few times. Despite the name "Babies" the character Bunny claims to have gone to high school and even had several wives (to which the character Puppy says, "Shit man, how old are you?").
In Darwin's Soldiers this trope is made explicit by Ariol, Kain, Subject 19, Siren, Breathtaker and Stalker; all have their age listed in their profiles as "unknown".
Most of the cast of Gaia Online qualifies, especially since the website does acknowledge the passing of years and the characters do have birthdays. They just don't resemble their alleged ages. At all.
Toki and her twin sister Doki are older than they appear but how old they are is never stated, though, going by Doki's Chronicles, they should be about either in their late eighties to early nineties, as the setting of one of the stories was in the nineteen twenties to thirties and their clothing from that point in time seems to suggest about that.
There is also Eglantine and presumably she is a toddler, as she is still in diapers and young enough to attend nursery school, though, if the passage of time is to go by, she should be four if not five and what is known is that she still a young child. Justified, in this case as no one has no clue as to how old Eglantine is because they don't know her birthday, thus they wouldn't know the year in which she was born in.
Brownieis stated to be "likely to be in early to mid twenties" though she was abou twelve or thirteen when she met Toki, making that a tad more apropriate.
Also, by the same one, we have Killerbunnies XI and XII: Kamu and Kanran who has a condition that makes them look like babies but they don't seem to let them stop them in being damn near malicious and that both are old enough to smoke but their ages are never stated.
The Histeria! Kid Chorus is stated to be in middle school (even the clearly teenage Toast and Pepper), but their exact ages are never stated (except for Froggo, who's ten).
According to Word of God, the ages of the main characters of Jimmy Two-Shoes are purposly left vauge in order to allow a bigger range of stories. Heloise looks like a little girl, but holds down a job as Misery Inc's top inventor, and all three live without parental supervision. Beezy can still get grounded and gets an allowance, but is old enough to get married.
Shego of Kim Possible apparently has a degree in teaching but (when she's not committing crimes For the Evulz) acts like a teenager, lounging around the Supervillain Lair reading fashion magazines. Her relationship with Dr. Drakken (who is Kim's father's age) flipped between father-daughter and romantic, even though she had a one-episode romance with Senor Senior Junior, who is apparently young enough to crush on Kim and date her high-school rival Bonnie.
In an episode when Shego was mind-altered to be nice, Kim referred to her as a "big sister", marking her clearly as a technical adult. Shego is probably in her mid-to-late 20s, for having a 27-year-old attracted to both a 22-year-old Junior (she acted like "the older woman" to Junior in that episode) and a 41-year-old Drakken would not be uncommon in real life, agewise. Word of God is that she was in her mid-20s by the end of the show, so basically between 24 and 26.
The ages of Kim and Ron (and by extension, their fellow students) are also never really addressed beyond Word of God describing them as "high school age". Although given a few details throughout the series (moving to senior year, being able to drive, graduating etc.) it can be reasonably assumed that they start out in the series at 14/15, before ending up at 17/18 in the final season.
Mickey Mouse and friends. Even in some merchandise where they are shown as school students, they still look the same.
SpongeBob from SpongeBob SquarePants. SpongeBob acts like a kid and is even called one, but lives in his own house, has a job, and is old enough to go to "boating school." It is mentioned several times throughout the show that he is an adult.
Actually, it was stated on his driver's license in the episode "Sleepy Time" that he was born on July 14, 1986, which would place him currently at about 28.
The title characters of Phineas and Ferb, as well as their same-age friends, were originally supposed to be nine years (as mentioned in the pilot), but when the creators saw how well various age groups responded to them they decided they were simply "less than fifteen" (their older sister's age). It's hard to get any clearer an example from how they act: Phineas is a Cheerful Child who's Oblivious to Love, while Ferb sounds like a teenager and has hints of being a Covert Pervert.
This also makes it hard to figure out if Ferb/Vanessa is just slightly improbable (two years isn't that big a deal!) or potentially pedophilic (a sixteen-year-old can't date someone who's nine!)
In his earliest cartoons, Porky Pig fluctuated between being a child and a fully-grown adult, the later easier to relate to from his future mild-mannered personality compared to other Looney Tunes characters.
The main characters of its successor, Tiny Toon Adventures, go to Acme Looniversity, which is likely to be a high school, but Elmyra doesn't look or act like a teenager, so how old they actually are is never stated, although they're most likely 13 years old as indicated by the title of one episode.
In The Raccoons, Bert acts like a child, but he also seemed to have graduated school and works as both a professional journalist and paperboy.
Spike in Transformers Generation 1 - when he first appeared, he was working on an oil rig, but in a later episode, he mentioned he was too young for a driver's license. In the same episode, he begins a romance with a girl who is in college.
Word of God says that Ruby Gloom is in her early twenties, but you'd never guess that from just watching the show.
Several official sources also state Ruby is five, though she doesn't really seem that young to most.
KaBlam!: Henry and June's ages are never stated in the show, but appearance wise, most likely nine to eleven, most fans put them at ten.
The titular character in Flip The Frog has a different age in different episodes. Sometimes he's portrayed as a kid, sometimes as an adult.
The members of Dethklok on Metalocalypse are of a vague adult age. In the first season, Pickles is said to have run away from home 15 years ago at the age of 16, which makes him 31. However, he also played in a hair metal band in The Eighties. If he was, say, 19 in 1985, that makes him 40 when the series begins.
Warner Brothers didn't want the writers to give Bruce Wayne an established age when he and Terry were celebrating his birth in the Batman Beyond episode "Out of the Past". They said in the commentary that he would have been around mid to late eighties.
Who knows how old any of the penguins in The Penguins of Madagascar are? It's worth noting that apparently Private is younger than the rest (but interestingly had at least one earlier identity as the Mini Golf champion Mr. Tux). We're given a clue that Skipper and company weren't alive, or at least were not yet adults, in the 1960s as Skipper wants to go back in time and slap a hippie. It's also worth noting that Buck Rockgut had been down in the tunnel under the zoo for forty-seven years when the penguins found him, and that it's implied that he's much older than they are. That's around twice a normal penguin's natural lifespan.
Scooby-Doo: The Gang are this incarnate. They're old enough to travel on the road on a consistent basis but still young enough to be considered "Meddling Kids." This would seem to imply that they're in the early 20's. Some episodes of the original series seem to place them in high school.
Word of God has Velma (the youngest) at fourteen in the original series and the oldest two being eighteen. In newer series they seem to have aged into their early twenties though, with some direct-to-VHS movies having them in their late 20s. However, in Scooby-Doo! Mystery Incorporated they're all explicitly still in high school, and though Shaggy and Velma's exact ages aren't specified Daphne and Fred are both stated to be Seniors.
The mane cast of My Little Pony: Friendship Is Magic seem to draw together adolescent and adult qualities. They all live independently and most work professionally, but also often act like teens (with things like slumber parties). Their interactions with explicitly pre-adolescent characters seems to vacillate between slightly older sibling to responsible adult figure. It's complicated even further by one minor who apparently has an explicit "teen as distinct from adult" stage.
The mane cast's age was given in one episode by showing Applejack as a baby at the Apple reunions (held every 100 months, with the reunion in the third season), leading to a large in-fandom argument about whether the ponies are 17 or 25. Their maturity level indicates the lower number, while their roles in society clearly requires the older, especially as that would require three of the main 6 to be business owners at 15.
Equestria Girls generally seems to hint that the Mane 6 are 17, as this was Twilight's age when she went to the human world. However, many child characters were age lifted as well to be freshmen as well.
Spike, who is both younger than the rest of the main cast but apparently older than the other children in the show. He lives with another character and basically acts as her younger brother, but unlike the other younger characters he can be left on his own for long periods of time (even taking over another's duties) and rather than going to school has a job assisting another cast member.
Throughout the series itself My Little Pony has fit this. Characters are explicitly referred to as adults and several of them even have children, however they're notoriously immature and love playing with toys. Then again they are horses.
Most of the time Donald Duck is portrayed as an adult, however one short "Donald's Better Self" depicts him as a child.
Alvin and the Chipmunks have cycled between being elementary school age kids, middle school kids, and high schoolers.
Betty Boop appears to be an adult or young adult, but in "Minnie the Moocher" we see her running away from home because her parents grounded her when she didn't eat her peas.
How old Piglet in the Disney versions of Winnie the Pooh is never specified. His name and the fact Tigger often refers to him by childish nicknames should mean he's young, but his voice and mature personality make him seem the same age as the others.
HBO Family sometime airs a show in the vein of Dora the Explorer called El Perro Y El Gato. The two protagonists seem like adults but in one episode the dog asks the cat what he wants to be when he grows up, and the dog says he wants to be a veterinarian.
Gwizdo of Dragon Hunters. In The Movie (which was a prequel to the series) he's established as a homeless adult; in the series he lives in an inn and has apparently been staying there since the innkeeper's daughter Zoria (who is around 18 now) was a child, which would mean that he's quite old. Yet appearance-wise he barely looks over 20 and in "The Isle of Mist" when he ends up in a Fountain of Youth for a few seconds he immediately reverts to a young child even though the monks before him had to bathe in it for several minutes just to revert to their thirties.
Jimbo, Dolph, and Kearney from The Simpsons. They are implied to be teenagers, but apparently attend Springfield Elementary. Kearney on the other hand, while he has a very screechy voice and acts like a teenage bully, he also has a son of his own, is divorced and according to Otto, they were in 3rd grade together.
Kearney, oddly enough, was shown in an early episode trying to buy beer with a fake ID and assumed name. Later, he's shown to remember Watergate and is also able to openly go for a drink at Moe's. Of course, the first incident was before they decided that his advanced age and apparent multiple failures to complete the sixth grade were a Running Gag.
Also, one episode shows that Jimbo is bald under his knit cap.
Chester McTech from Beverly Hills Teens. He's younger than most of the cast, but by how much is never specified.
It's hard to pin an age onto the Strawberry Shortcake characters. They look like preteens both in their 2003 and 2007 designs, despite the fact Apple Dumplin' aged noticeably at least six years. It's made even more confusing by the fact the later episodes involve the characters getting jobs, moving out, and driving cars. The 2009 is equally bad, with the characters owning their own stores but looking no older than fifteen.
Ami and Yumi from "Hi Hi Puffy AmiYumi"', they are assumed to be adults like their real life counterparts but the art style makes them look like children and they act like teens.
The Popeye character don't have their ages revealed, though "Goonland" and "Popeye, the Ace of Space" imply that Popeye himself is around 40, while "The Man on the Flying Trapeze" has Olive being called "a maid in her teens". Also in "Puttin' on the Act" (from 1940), it's shown that Popeye and Olive had an 1890s vaudeville act.
Total Drama has a mild example with Blaineley, who's clearly supposed to be an adult, but winds up competing in the show with teenagers. She gets Ship Tease with both adult Chef and teenage Owen, and admits to having a crush on Justin.
In The Beatles cartoon, the Fab Four themselves are in their moptop-era designs, meaning they're meant to be very young adults. However, a few episodes have them refer to themselves as teenagers, and yet another episode has them at a legal drinking age.
It's never stated how old the human main characters of The Grim Adventures of Billy & Mandy are supposed to be, and their school seems to combine elements of elementary school and junior high.
If you're the type of person who doesn't ask people for their age, this is bound to happen.
Both BlueStahli and Celldweller. Due to the fact that they never mention their age nor are their ages listed on any website.