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 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 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.
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).
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.
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.
Weijuaru of Juathuur. The thing is... he's a shift user.
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.
Vinci, of Vinci and Arty, is often mistaken for a child, in spite of being well into adulthood.
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.
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.
Nina Delacroix of Eerie Cuties looks and acts like the comic's Token Mini-Moe, but she's the same age (14) as her friends and in high school.
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.
Sera from Serenity Rose is extremely short, and the art for the comic is fairly age-ambiguous, to the point that the artist has on at least one occasion included a disclaimer stating she's in College.
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.
Precocious: Tiffany in this request strip showing the kids as teenagers.
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.
Magic users in Evon age slower than normal people. Evon looks like a twelve-year old (allegedly) but is actually in her mid-20s.
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.
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.
As of the Credomar arc, Kaff Tagon is 49. This is brought down in particular by Elf, who in this strip claims he's older than her dad, and yet she had a crush on him a few arcs earlier.
Worse is Alexia Murtaugh, who's military experience stands above Kaff's, and is third only to those of Karl Tagon and Thurl.
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.
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.
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.