City of Trees makes a point to call attention to the fact that many magical character have much longer lifespans then their appearances would suggest.
Amir: Seriously, Jas, he probably only looks fifteen. I bet you anything he's really older than this whole city.
Mana Ryushin, from Random Encounter and Way Back Home, initially appears as a young child, despite having lived hundreds of years in the afterlife. Eventually she gives herself an adult body, which still doesn't look a day over thirty.
Dina from Dumbing of Age; college-aged, but about half the size of her classmates and lacking in social skills. Lampshaded when Raidah and her friends think she's a child, and later after she makes friends with Riley.
Amber: Dina, I think she's like twelve. Dina: I have been deceived! Riley: Wait, you're not twelve? Dina: I see no answer to that which would not tarnish my dignity.
Mr. Raven of El Goonish Shive looks fifty-ish, but he's old enough to have taught a current student's mother (herself about forty). At one point, we see him unconscious, and he actually looks younger, implying he has some sort of illusion spell going to make himself look older. Oh, and he's an elf.
Grace is 18. Not all that implausible, but this has caused some in-story surprise when people found out that she's the oldest person among the main cast.
In Errant Story: Meji, age seventeen, looks twelve (she's a half-elf). All kinds of angsting and insecurities ensue. Fellow half-elf, Ian, is 45 and looks like he hasn't hit 20 yet.
Zigzagged with Nina Delacroix, who's initially pint-sized and acts like a 12 year old, despite being 14note from Vol.0 until the start of Vol.3. It's eventually revealed that her growth was being stunted because she refused to drink blood like a proper vampire. But once she finally does, she grows a few inches taller and now looks the same age as her friends.
Played straight with Tia Darkness, who appears to be the same age as Nina once did. However, there were subtle hints that she was older than she let on, such as when she referred to Nina as, "little one". While Tia's actual age is never specified, the "Tia &Me" epilogue short story finally confirmed that she's at least several hundred years old, if not thousands. According to ancient historical texts Tia is actually Dementia Oblivion, a grand chaos demon and sister to both Artemis and Hecate from Magick Chicks!
Magic users in Evon age slower than normal people. Evon looks like a twelve-year old (allegedly) but is actually in her mid-20s.
In Frivolesque, the reveal that the small, child-like Flore is slightly older than her much more mature looking sister comes off as a surprise. This seem to apply to their father as well.
Girl Genius: Embi, a member of Master Payne's Circus of Adventure, is apparently well over a hundred years old.
By taking the function of makeup Up to Eleven, this is played straight with Song Chohyang in Gosu. Normally she looks like a small elderly woman with wrinkles and grey hair. However, once she applies her makeup, she quite literally transforms into a beautiful young woman. Chohyang's makeup skills are so transcendental that she made a mother unrecognisable to her own children.
Jones of Gunnerkrigg Court looks like she is around thirty. She has always looked like that. Always. She is actually billions of years old and no more human than a rock is.
Homestuck: Dave's Bro looks to be in his early to mid twenties, and the age gap isn't too wide between him and Dave, in reality he's as old as Rose's Mom as a Paradox clone sent to earth on a meteor, he arrived at the same time she did making him in his thirties-forties.
The heroines of Jet Dream are chronologically in their 30s or 40s, but appear to be young women in their early 20s. When the former men of the Thunderbird Squadron were turned into girls by Virus-X, the effect also rejuvenated them.
Weijuaru of Juathuur. The thing is... he's a shift user.
Miho of Megatokyo has stated outright that she's older than she looks (about seventeen), though Largo thinks she's ancient. She also refers to a twenty-three year old as "young man" and used to run around with Yuki's mother, Meimi, which pushes her estimated age closer to forty.
Nearly everyone in Oz in Namesake. The spell placed on Oz prevented the inhabitants from aging, meaning only those who really wanted to age did so. Ozma in particular appears very young, in her younger teens, despite being however old she really is. In a horrifying twist on the trope the Dorothies don't appear to age physically, but die of old age anyway, to Ozma's despair.
Nebula: All of the characters' ages veer into Time Abyss, but Earth and Sun have Older Than They Look apply within what's normal for their species. Earth is stated to be stated to Venus' older sister, despite not looking any older than her and having more than a bit of Immortal Immaturity. Sun, meanwhile, is mentioned by the other characters to be have existed since before anyone else did, even though he looks about the same age as them.
Ozy and Millie: Locke is actually Millie's father and in his world, everyone ages backwards. He's actually just a few years away from death.
Precocious: Tiffany in this request strip showing the kids as teenagers.
Angela from Queen of Wands and Punch an' Pie. She doesn't seem to mind it much, because hey, ordering off the kid's menu is cheaper and you get to color on the placemats.
Torg and Riff of Sluggy Freelance had been old enough to drink for multiple years (They had been giving each other beers for Christmas) at the start of the comic in 1997. Their comic makes note of major holidays, making Comic-Book Time and real time correspond to a rough degree. But despite the fact that those two should be close to 40 years old now, neither they (nor anyone else in the strip) has noticeably aged.
Spinnerette looks like a teenager but she's actually a grad student in her mid- to late twenties. This contrasts with "Evil Spinerette," who looks considerably older than the 16 years she turns out to be.
Tiger: Wow, kids grow up fast these days. Mecha-Maid: You talking about the supervillainy or the ginormous boobs? Tiger: Both, I guess.
Da-Jeong from The Friendly Winter is a nineteen year old who looks like a seven to eight year old. She has a thyroid condition that stunted her growth. She could have gone on estrogen to have her grow but by the time they figured out it was too late. Her mother abandoned her ten years ago because she couldn't handle her daughter's disability. Da-Jeong hates that she's forced to masquerade as a little kid (including wearing childish clothing and having a kiddie bike) and no one believes her when she says she's an adult.
Greg, boyfriend of Kili in The Dragon Doctors, is really 60 but looks about 20-something. However, he's not alone, as apparently quite a lot of people are able to purchase rejuvenation in the setting. Mori takes it further by being 170.
Pompey the half-elf is approximately sixteen both mentally and physically; chronologically, he's forty-three, and complains about being called "kid" when it's revealed that he's taking orders from a (human) 21-year old.
Vaarsuvius has two elf children, aged twenty-six, who look and act like small kids and are currently in kindergarten. Both of these are the result of long elflifespans.
Redcloak still looks like a goblin teenager thanks to the Crimson Mantle despite being over fifty (which, for a goblin, is beyond venerable age).
Stand Still, Stay Silent: Tuuri is 21, but is also quite short for an adult, has youthful looks, and an enthusiastic personality that can be accurately described as child-like. When someone she just met recently tries to guess her age, they go for 16.
Tower of God is an undisputed king of this Trope. Manipulating the Functional Magic of their world known as Shinsoo, allows for a person who becomes skilled enough with it to become nigh immortal. Since the entire cast and everyone in the the story setting can use Shinsoo, almost all of them are already much older than they look. A prime example is Anak Zahard who has a body of what seems like a child yet is over 300 years old. They need to be nigh immortal as well, since their ultimate goal is to reach the top of the Tower that makes up their world and just reaching the 134th Floornote The highest anyone has ever gotten takes the average person 500 years.
Vinci, of Vinci and Arty, is often mistaken for a child, in spite of being well into adulthood.
Sock from Welcome to Hell is officially fifteen-to-twenty however he looks more like a ten-to-thirteen year old. He's short, rather androgynous, and in the animated short is voiced by a girl.
Sung Sumin from What Does The Fox Say is a thirty four year old that looks significantly younger. The protagonist doesn't even believe her age at first. Sumin's age is a hugeBerserk Button for her, as in the past she was disrespected for looking young, so the protagonist innocently questioning it makes their relationship start on rocky terms.
Both Max and Ivan from Younger Days fit this trope. Max is 9 but looks more like a toddler aged about 4, and Ivan looks like he might be 7 but is actually closer to 11.