Humans of Númenórean lineage, such as Aragorn, physically age slower and live longer than normal men. Aragorn, for instance, turned 88 the day he met Gandalf the White in Fangorn Forest. A Deleted Scene in the film of The Two Towers has Aragorn reveal his true age to Éowyn, much to her surprise. (The book reveals that he lives to about 210.)
Possessing the One Ring also slows down aging; for the 60 years Bilbo carries the Ring he does not appear to age much at all (but it catches up with him after he abandons it), and for the 17 years Frodo has the Ring he experiences the same thing.
Alongside Bilbo, normal Hobbits also age slowly - Frodo was 33 when he took up the Ring and considered to have only just come of age, while Pippin was 29 during the War of the Ring and is mistaken for an early teenage male. When Aragorn and Legolas ask Éomer if he did not find Hobbits among the slain Orcs, they describe them as small bodied and child-like in appearance. There are many short people in Real Life, but they do not look childlike. By comparison: Frodo is 10 years older than Boromir, and Sam is only two years younger.
In Teresa Edgerton's second Celydonn trilogy, Gwenlliant is initially twelve years old (in Castle of the Silver Wheel), and looks it, but thanks to Time Travel is at least a year older than her official age by the time she is technically fifteen (in The Moon and the Thorn).
Gets a passing mention in David Weber's Honor Harrington series, where recipients of prolong treatment have a very extended physical adolescence. A visitor from a planet that doesn't have it is disturbed at seeing a combat warship full of what looks like teenagers and even pre-teens. It's also mentioned in passing that sex among crew members is something that happens fairly regularly, so...
Even newborn babies are Older than They Look: for a mother who's had prolong treatment, pregnancy lasts eleven and a half months.
The trope also occurs in Weber's Empire from the Ashes, as a combination of biological enhancement, suspended animation, and (among the bad guys) outright body surfing.
In the Discworld novel Mort, we find Ysabell embodies this trope. She is 16, but has been 16 for 50 years, since she lives in Death's Domain, where time does not pass. We later find Albert embodies this trope.
Most of the characters in John Scalzi's Old Man's War, thanks to them being the minds of elderly humans copied into the bodies of young mutant supersoldiers. It only makes the ensuing orgy scene a little bit squicky.
In Garth Nix's Lirael, the 19-year old title character senses her travelling companion Sam (who turns out to be her nephew...) developing a romantic interest in her, so she tells him she's thirty-five with a marvelous skin-care regimen. Her Nonhuman Sidekick, the Disreputable Dog, backs her up because she thinks it's hilarious.
In the latest novel in the Ciaphas Cain series, Cain's Last Stand, Cain is a nearly 100-year old retiree who appears to be in his early fifties or so thanks to regular juvenat treatments. Characters who have been receiving the treatments longer can cross into Really 700 Years Old, such as Alizabeth Bequin in the Eisenhorn series, who's 170 or so but doesn't look any older than her 20s.
In The Chronicles of Narnia, the four Pevensies look like children, and are as far as anyone in their own world is aware, but they are in a sense technically adults because they spent fifteen years in Narnia. Although they did de-age once they went back in England, they are the only people who have gone to Narnia and grown up. This idea is more played with in the fan-fiction universe.
In the short story Start the Clock by Benjamin Rosenbaum, a virus that caused humans to stop physically ageing struck the Earth around thirty years ago, making the protagonist, physically nine years old, technically thirty-nine. An interesting facet of this is that characters who are physically children tend to act like children who have had around thirty years extra experience at being them. Then later in the story, the protagonist meets a girl who is mentally the same age as she is physically, having been infected by the virus at the age of two and only just recently been administered a "cure" to allow her to age naturally.
In Stephen King's The Stand, Tom Cullen is said to be looking no more than twenty-three; actually he's at least forty-five. The fact that he acts like a kid probably doesn't help.
Several of the secondary adult Harry Potter characters qualify here, in particular Dumbledore and Griselda Marchbanks, each of whom are well over a hundred years old; it's implied that wizards have a somewhat longer natural lifespan than muggles. Voldemort has also ceased aging altogether, being 71 years old by the last book, though his monstrous appearance hardly seems like a fair trade for smooth clear skin.
Although he still looks pretty old, Jack Holloway in H. Beam Piper's Little Fuzzy novels is technically 10 years older than he appears physically due to the time dilation effects of hyperspace.
In The Picture of Dorian Gray, the character of the title transfers the effects of age and wickedness to a portrait of himself. After twenty years, he hasn't appeared to age a day.
In Jim Butcher's Codex Alera series, watercrafters have an innate healing ability which causes them to retain their youthful appearances even as they age — Gaius Sextus, for examples, is over 80 but looks around 40. A scene in the fourth book has a cop mistaking the 50-year old Isana for a streetwalker, until she lifts her hood so he can see her graying hair.
Mention should also go to main character Tavi. In the first book, Amara guesses he's twelve or thirteen, only to be informed that he's fifteen. Thing is, Isana's been lying to him about his age: he's actually almost eighteen, since he was born the night Septimus was killed. Granted, his apparent youth is due to her magically stunting his growth.
In Jim Butcher's The Dresden Files novels, when Captain Luccio is body swapped with a 30 year old during a battle, all kinds of interesting complications emerge because the person definitely fits this trope, since the individual was over 200 years old before the swap.
Most of the Wilds in Trudi Canavan's The Age of the Five trilogy and most of the The White and The Voices are a lot older than they look. Juran, the leader of the White, is actually over 100 years old, his aging having been stopped by the Circle of Gods when he was chosen to be one of their human representatives. They only get older from there.
Lakewalkers in The Sharing Knife books tend to be this to some degree. When Fawn Bluefield brings her rescuer/beau Dag Redwing home to her parents her father starts an interrogation about his intentions with a question about his age (everyone, including 18-year old Fawn herself, takes him to be somewhere in his thirties) over the dinner table. When Dag answers (with some hesitation) "fifty-five", Fawn promptly chokes on her cider.
Fawn:(whispering) Pa... is 53!
Also Fawn herself. Her entire family is short, and she is best described as petite. She is constantly mistaken for a child and twice the mistake is only realized by seeing or touching her breasts. The misunderstandings increase when she is standing next to the two heads taller Dag.
John Geary, the protagonist of Jack Campbell's The Lost Fleet series, was a Human Popsicle in a lost escape pod for over a century. After his rescue and revival, his "posthumous" promotion to Captain gives him seniority over every other Captain in the fleet — which he exploits to pull their collective chestnuts out of the fire, whether they like it or not.
The protagonist (and questionably sane narrator) of The Tin Drum, Oskar Matzerath, throws himself down the stairs at age three to stop growing. Over the years he exploits his young appearance to get (and get away with) whatever he wants, at the expense of those around him. He forces himself to start growing again when he's in his twenties, after his father dies.
In Percy Jackson and the Olympians, Nico di Angelo looks twelve. He's actually seventy-something from being put in the Lotus Casino, which slows down time. A lot. His sister is also the same. Being set in a world where Greek Mythology is true, many of the other characters are at least 2000 years old. Grover, on the other hand, is thirty-two but looks sixteen (satyrs age slower than humans).
In Peter Watts's Rifters Trilogy a pedophilic character is captured by a police sting operation which used a man who had been artificially transformed into an apparent child as bait.
In the Alliance/Union novels by C. J. Cherryh, FTL travels slows down physical aging by around 25%. So spacer teenagers look like middle school kids, twenty-somethings look like teenagers, etc.
Legacy of the Aldenata: All rejuvenated personnel qualify for this, appearing to be in their early twenties or younger, while dating back, in some cases, to before World War II. It's occasionally an issue until people start assuming that anybody who looks 20 is probably older. Often a lot older. And you get generation bending, Michael O'Neal Sr's second wife is almost assuredly younger than his son (her age isn't stated, but comparing the children's ages leads to this conclusion).
In Stieg Larsson's Millennium Trilogy, lead character Lisbeth Salander is mentioned as looking "barely legal", which in Sweden is 15. She's actually 24 at the time of the first book, but is described as an "uncommonly thin and fine-boned" person.
The title character in Arthur Machen's short story, "The Bright Boy", looks to be about 7-10 years old, but is actually a man in his 60s. The ending reveals that he was likely conceived by a mortal man and an infernal woman. It also reveals that he is a fugitive rapist who is passing himself off as the son of a wealthy couple. Because of his abnormal nature, he stands out like a sore thumb and there is no other way for him to hide from authorities.
In P.N. Elrod's vampire novels, vampires who are older than about 20 revert to that age when they die and return. This was most evident with the once-elderly Gaylen in Lifeblood, and is a frequent annoyance to Jack Fleming, who's chronologically in his late 30s yet is regularly called "kid" by people who don't know this. It also means Jack may never be able to see his parents again, as he doesn't want to tell them he was murdered and his newfound youthfulness would demand an explanation.
Scourge of Warrior Cats, mostly because of his small size. Firestar even mistakes him for an apprentice at first. His name was Tiny when he was a kit, to emphasize this further.
From the Left Behind books: once the Millennium starts in Kingdom Come, all the naturals who enter this time period experience decreased aging similar to the first several generations of mankind in the book of Genesis, with the children becoming young adults by the time they reach 100. It's not explained how those who were already adults, including those who were already at advanced age, experience this decreased aging at the same rate as the children who enter the Millennium. By the end of the Millennium, however, the longest-living naturals end up really showing their age.
Geralt of Rivia, the titular character. Reading between the lines, you can see that he is well into his eighties, and has white hair. However, the mutations he has been exposed to and his active lifestyle give him the appearance and physique of an extremely fit man in his mid-twenties.
Geralt's friend, chronicler and drinking buddy Dandelion has been described as being forty, sounding like he's thirty, looking like he's twenty and acting like he's ten.
Vesemir, Geralt's mentor and the oldest witcher alive, may be pushing 800 but is still a match for Geralt physically.
Wild Cards has this with several characters, most notably the Golden Boy, Jack Braun. Possessor of Nigh-Invulnerability, he's an original, who gained his ace powers when the virus was first released in 1946, and is near immortal. Also applies to the Sleeper, Croyd Crenson (Also an original, though since his appearance is altered every time he wakes up how much this applies is debatable), and Dr. Tachyon, who is described as looking like a man in his 20's or early 30's, despite being in his 80's (The Takisian species ages differently from humans).
Nicholas Nickleby: The Crummles' daughter is a young adult but looks like a little girl and is claimed to be 10. This turns out to be deliberate as her parents deliberately stunted her growth with gin so that she could continue to pose as a child in their stage act.
In Robert A. Heinlein's The Moon Is a Harsh Mistress, Lunar colonists age more slowly due to the lower gravity. In fact, after a hundred years, no one born on the Moon has yet to die of old age, so they don't even know their own life expectancy. When an Earth reporter guesses Manny's age as twenty-two, he thanks her for the compliment and states that he's been married longer than that. Manny's appearance and Loonies' slow aging aren't mentioned until the middle of the book, so the fact that Manny looks so young is a surprising reveal.
Robert A. Heinlein's stories about members of the Howard Families. These individuals appear to age at a normal speed until maturity, then slow down drastically (easily passing for forty when in their seventies, for example). After rejuvenation therapy is developed, someone who looks eighteen can easily be twenty times that age or more.
Doc Sidhe: As a pureblood sidhe, Doc ages very slowly. Harrison's initial estimate of Doc's age is off by several decades.
In the Belgariad Belgarath looks 70. He's 7000. Proportionally beaten by his daughter Polgara, who looks 30, and is about 5000.
Ceasar Flickerman has been hosting The Hunger Games interviews for over 40 years, but doesn't look much older than when he started because of extensive plastic surgery, a very common practice in the Capitol.
Early in the first installment of The Hitchhiker's Guide to the Galaxy, it's revealed that Zaphod Beeblebrox is about 200 years old, which means also that Ford Prefect is as well, since they were childhood friends. It's only mentioned in passing, though, and doesn't really matter much to the plot.
Lady Gray, from the web serial novel The Graystone Saga, has the appearance of a woman in her late twenties. Her exact age has yet to be revealed, but a few references have been made to her being older than she appears; for instance, she mentions having been present for an event which took place prior to the birth of her confessor, Father Matthias, who is at least sixty.
The Eloi, the above-ground people in The Time Machine, resemble androgynous five- or six-year-old kids, both physically and mentally. They look five and act five, even as adults. Chapter 4 implies that they stop maturing early, before Secondary Sexual Characteristics have much of a chance to develop.
In the Red Mars Trilogy it's stated that repeated frequent use of the longevity treatment can lead to this. One character is described as looking like she's in her twenties but is really in her nineties. The main characters tend to avert this since they typically only take the treatment once every few decades or so.
It's tough to gauge the ages of the alien Tendu in The Color Of Distance, since they look like frogs built like spider monkeys. Still, one of the mains has just metamorphized into full adulthood in her sixties, the usual age which Tendu attain is around 120, and while they get older they don't age. Ukatonen is over six hundred and says there are other enkar ten times his age and even they are not the oldest of the Tendu. Most Tendu commit suicide rather than leave home and become enkar, and Ukatonen admits to having considered killing himself out of boredrom, nothing new happening in such a long time, until humans arrived.