An episode of Law & Order: Special Victims Unit features a girl named Janie Spear with Turner's Syndrome, so even though she is seventeen years old, physically, she looks like a 12-year old child. The episode focuses on her relationship with a 30-year old employee of her father's; since she's seventeen it isn't statutory rape, but none of the detectives are comfortable with what this says about her boyfriend's taste in girls.
And in another episode, a girl in her late to mid-twenties not only poses as a 16-year old, but is under the delusion that she is, despite having a complete memory of all the years she has spent going from one foster home to another and creating false identities. Based in the real case of Treva Thornberry.
In another episode, there was a young rookie cop who took down a suspected rapist until it was found out that the cop was really a 16-year old wannabe.
An episode of Law & Order revolves around a teenager who is arrested for the murder of one of her teachers, followed up by another teacher arrested for sleeping with her. In actuality the "teen" is a con-artist who is in her thirties, and killed the teacher to prevent her from revealing her real age.
In one episode of CSI: New York, there is a subplot where one of the suspects was a victim of the real culprit, a wanted sex offender in his twenties posing as a high schooler. Compare to the examples above.
The 1980s hit Twenty One Jump Street was entirely based around the premise of cops who looked young enough that they could go undercover as high school students.
In the Star Trek: The Next Generation episode "Rascals", Keiko O'Brien is temporarily turned into an adorable 12-year old. The fact that she still wants to be treated like a wife squicks Miles out a bit. Three other crewmembers - Picard, Guinan, and Ro - are similarly altered, but none of them are in relationships at the time, though they all happily exploited this trope to retake the ship from a hostile Ferengi takeover.
The O'Briens seem to get more than their fair share of this trope. There's another Deep Space 9 episode, "Hard Time", where an alien race sentences O'Brien to a 21-years-long virtual prison sentence. He experiences all 21 years of his brutal, tortuous prison term in only a few days, making him mentally 21 years older; 21 really, really crappy years older. And even their daughter gets a Overnight Age-Up treatment due to a Negative Space Wedgie.
In an episode of the new The Outer Limits, a prison was created where prisoners serve their prison sentence within a few hours. The creator of the prison is trapped inside and serves a 20 year sentence within a few hours, which would mentally make him this trope, like O'Brien above.
In Firefly, Malcolm Reynolds looks like he is, at most, in his mid-thirties. However, a comparison of the dates of his birth and the dates of the "present" indicates he is in his late forties to early fifties, which is supported by Mal referring to himself as a "mean old man." This can be justified by the advanced medical technology of the setting, however, or perhaps by Joss Whedon originally envisoning the character as being played by an older actor.]
Jesse Travis of Diagnosis: Murder got this on occasion. How gracefully he reacted depended on how snotty the other person acted.
Nearly all the regular vampire characters in Buffy and Angel are a century or more old but look the same age as when they became vampires. This trope applies to vampires in general, really, though very few in the Buffyverse seem to make it past their first hundred years without getting staked.
In the future dystopia of Dollhouse, "Iris" seems to be a 10-year old girl but actually has the imprinted mind of a grown woman — first a woman who pretends to be a girl before pulling a gun on Zone, and later Caroline, whom Zone and Mag upload into the girl to find the way to Safe Haven to avoid the mass mindwiping that has thrown civilization into chaos.
Babylon 5 novels reveal that Alfred Bester is in his seventies by the time we first see him.
In 30 Rock, it's implied that head page Kenneth is lots older than his callow youthful appearance would suggest.
Tracy: Ken, you don't want to be a page forever? Kenneth:(alarmed) Who said I've been alive forever?
During Degrassi's first Big Damn Movie, while Paige is living up the diva socialite lifestyle, she has herself an 18th birthday party. Which could be chalked up as Blatant Lies. The actress is in her mid twenties and the character is in her early twenties. It also can be counted as a casual jab at Dawson Casting.
Clare's schoolgirl attire and hairstyle does a good job of dropping her apparent age from 14 to a tween. Probably as a means to play up her innocence. Her pink outfit in Causing a Commotion has a similar effect, as this before her wardrobe upgrade.
An episode of Glee had one character stating that Kurt (who was about 16 at the time) couldn't have purchased alcohol with a fake i.d. because he looked "like a 12-year old milkmaid." She's exaggerating, but not by much◊. Chris Colfer himself was 19 at the time of filming.
In the original performance of Cinderella, 30-yearold Edie Adams portrayed the centuries-old Fairy Godmother.
Used in Lost In Oz as a plot point. Princess Ozma is said to be in her 20s, but was enchanted to be an 8-year old forever.
Juken Sentai Gekiranger's Gou Fukami looks like he is in his mid to late 20s, but in flashbacks to his brother's childhood looks exactly the same age as he does now, and dialogue implies that he is or should be around the age of the 40-something Miki. It can possibly be implied that he stopped aging physically after turning into a werewolf.
A similar example is Masato Jin in Tokumei Sentai Go-Busters, who is 40 years old but due to his physical body being stuck in hyperspace can only appear as an avatar of himself at 27, the same age he was when transported to hyperspace. His body, which is preserved in a chamber there, also hasn't aged.
Power Rangers SPD gives us Dr. Kat Manx, who looks to be in her mid-twenties, but is actually one hundred and forty-seven Earth years old.
In one episode of Boy Meets World when Eric goes to Hollywood to work on a sitcom he meets an actress who plays a little girl on the show but is actually 42 years old. She then hits on him by grabbing his ass.
In one episode of Smallville, Lana Lang is haunted by the ghost of a childhood friend, Emily. Emily was stated to be 10 years old, but she looks and acts much younger, with handwriting that looks more like a 2nd grader and interests and speech patterns more typical of a 4 or 5 year old. In flashbacks, Lana appears to have been an improbably immature 10 year old as well, since she didn't act very differently from Emily.
This trope is a plot-point in the Doctor Who episode The Doctor Dances. The Doctor deduces that Nancy (who looks to be about 16) is actually in her early twenties (and therefore, the Child/Jamie's mother, not his sister).
Although he more accurately falls into Really 700 Years Old when it comes to apparent age vs actual age (he's around 900), the Doctor on Doctor Who also occasionally falls into this trope when it comes to apparent age vs mental age: for example, the Eleventh Doctor appears to be his mid-20s, but mentally is more like a wise and learned (as well as cranky, shell-shocked and somewhat senile) elderly grandfather. And this is actually implied to be inverted with the First Doctor, who looks to be in his 60s (and puts up a somewhat convincing front), but is mentally closer to a rebellious, immature youth who's sticking it to The Man.
Damon Razor, a hot draft prospect in Necessary Roughness, is eventually revealed to have assumed the identity of a deceased boy four years younger than him. After re-enrolling in high school, he became a star football player as his short statue was no longer a factor with the younger team.
While Stargate SG-1 has plenty of Really 700 Years Old for many races, the Jaffa fall more under this trope due to their more realistic lifespans. Free of disease and mutations thanks to symbiotes, they can continue to serve as warriors well into their second century of life while looking quite youthful.
Lampshaded by Clive James in a 1987 TV special where he visited Japan. In one of Japan's famed hot springs, he converses with a salaryman who gives his age as 38. To which James responds, "You're 38, and you look 20. I'm 47, and I look 87."
''The X-Files: Eugene Victor Tooms from "Squeeze" and "Tooms" was a genetic mutant and Serial Killer who needed human livers to hibernate. He looked to be in his late twenties in 1993, but his first murder had occurred in 1903.
Alfred Fellig a photgrapher who unfortunately gained immortality by tricking Death from "Tithonus".
Once Upon a Time: Thanks to a curse, all of the fairy tale characters were frozen in time for 28 years, meaning for example that Snow White and her daughter Emma look like they're the same age. Since when the curse is broken they seem to have all of their memories of those 28 years, they're technically mentally thirty years older than they look.
Aaron Samson, a one-off villain from NTSF:SD:SUV. He appears to be in his late 40s, but a photograph from 20 years ago shows that he hasn't aged a day since 1992. NTSF:SD:SUV agents variously theorize that he's a robot, since his father was a famous roboticist, or that he's a vampire, based on the suggestion of an FBI paranormal investigator. It turns out his mother was Asian, causing him to age very gracefully.
Niles the Butler in The Nanny. Though he looks the same age as the other main characters, he's roughly 20 years older than the title character.
In Supernatural, Sam and Dean have both aged mentally several decades beyond their physical ages. Dean spent around 30 years in Hell, which means that he's mentally in his early 60s. Sam spent an equivalent period in Lucifer's Cage (it's not explicitly stated how long though). This is not touched on much in the series.
In Earth2, hotshot pilot Alonso Solace says he's spent more time in cold sleep (en route to planets) than he has awake, making him much older chronologically than the twenty-something he looks.
Danziger (in his late thirties): So, how old are you, then? Alonso:A lot older than you, kid.