In another episode, they find out that Tracy is older than he looks. This leads to a discussion about the belief that African-Americans tend to look much younger than they actually are ("Black don't crack").
Arrow / The Flash: Barry Allen is 25 years old in the first season, but looks like a teenager. Ollie repeatedly ribs him over this, such as jokingly telling Felicity that he's instructed the bartenders at his party to ask Barry for an I.D. before serving him drinks. This is due to Barry being a rare example of not using Dawson Casting. Grant Gustin is in his mid-twenties, and so is Barry.
Babylon 5: The novels reveal that Alfred Bester is in his seventies by the time we first see him.
Boy Meets World: In one episode, 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.
Nearly all the regular vampire characters in both shows 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 Season 9, Giles' aunts use magic to prevent them from aging.
Anya is at least a thousand years old, yet is stuck in the body of a teenager.
Clive James: Lampshaded by him 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."
CSI: NY: In one episode, 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.
During its 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.
Diagnosis: Murder: Jesse Travis got this on occasion. How gracefully he reacted depended on how snotty the other person acted.
This trope is a plot-point in the 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 claims to be 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.
Dollhouse: In the future dystopia, "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.
Earth 2, 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.
The Cardinal Witches from Emerald City are far older than their appearance would indicate. West in particular, played by a 31-year-old actress, participated in the last battle against the Beast Forever 20 years ago and was already having sex with Anna's mother before Anna was even born.
In Episodes, actress Morning Randolph looks like she's about 30 at the most, and is playing a character described as "mid-twenties", but thanks to extensive plastic surgery, looks much younger than she really is. She did a pilot in the 80's with William Shatner, wherein she played his wife, implying she's probably well over 50. When Sean sees her age written down (we don't get to see it) he reacts with shock and says to (late-40's) Beverly, "She's even older than you!"
The show is about a sitcom focusing on a high-school hockey team. The actors playing the kids are in their late 20's, and one is said to be in his 30's. This is true both in-universe and real life.
Famous in Love has teen movie star heartthrob Rainer told he can't do a stunt because he's only 20. Rainer responds he's really 23 and his mom/agent Nina "youthed me down" when he started out. This is the first clue to Alan figuring out that Rainer is his son.
Subverted as Nina has been having an affair for years with Rainer's best friend Jordan who she met when he was starring in a teen series, assuming he was the classic case of a twentysomething posing as a teenager. When Jordan's girlfriend, Tangey, finds out, she blackmails Nina to get her career going. When Nina first refuses, Tangey reveals that Jordan lied about his age to get that role and was really only sixteen when he and Nina began their affair, meaning Nina goes to jail if the truth comes out.
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 might be because of the advanced medical technology of the setting, however, although perhaps Joss Whedon originally envisioned the character as being played by an older actor.
Forever has Henry Morgan, who is about 200 but looks to be in his 30s.
Game of Thrones: In the season 6 premier, the young and beautiful Melisandre is revealed to be an old crone who hides her appearance with her magical ruby choker.
Glee: One episode 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.
House of Anubis: Victor looks like he's in his 50's while he's actually 95, thanks to the elixir he took. Same with Rufus, who looks about 40 but is also 95. Similarly, Robert Frobisher-Smythe looked about 50 when he was released from the tank, but was really over a hundred years old. When the curse was broken, he started to look his actual age.
Juken Sentai Gekiranger: 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.
Kamen Rider Double: Akiko Narumi is 20 years old, but looks much younger. Shotaro even mistakes her for a middle schooler in the first episode, and continues calling her that as an insult throughout the course of the show.
Highlander: Upon surviving a serious injury, or "first death", an Immortal can become multiple-millennia-old and will still look like the same age they were when they became Immortal and can only be killed by decapitation. For example, Kenny is 800 years old, yet he is physically a child due to this.
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.
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 is fine but some of the detectives are uncomfortable with what they think 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 Thorneberry.
Lost In Oz: Used as a plot point. Princess Ozma is said to be in her 20s, but was enchanted to be an 8-year old forever.
The Nanny: Niles the Butler. Though he looks the same age as the other main characters, he's roughly 20 years older than the title character.
Necessary Roughness: Damon Razor, a hot draft prospect, 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. In this case, this was Ripped from the Headlines, based on the case of high school basketball player Jerry Joseph.
NTSF:SD:SUV::: Aaron Samson, a one-off villain. 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.
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.
Various other characters in the series fit this trope as well. Rumpelstiltskin is hundreds of years old due to acquiring immortality when he became the Dark One. Physically he looks no older than a man in his 40s or 50s. Captain Hook, Peter Pan, and other inhabitants of Neverland are also centuries old while retaining a youthful appearance.
The Outer Limits (1995): In the episode "The Sentence", 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 feel like he fit this trope.
Power Rangers S.P.D. 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 Power Rangers Dino Charge, this is one of the perks of being bonded to an Energem. As a result, Koda looks like in his 20s, but is actually 100,000+ years old. James Navarro, who is Tyler's then-missing father, looks ten years younger than his actual age. The villain Heckyl is later implied to be millions of years old.
Revolution: Nate Walker might look like twenty-something, but the actor playing him, JD Pardo, is actually 33 years old. Tom Neville looks like he's in his forties, but the actor playing him, Giancarlo Esposito, is actually 55 years old. Captain Riley looks like he's twenty-something, but the actor playing him, Omid Abtahi, is actually 34 years old.
Smallville: In one episode, 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.
She Spies had them investigating a shady charity and one member getting friendly with a little girl in a wheelchair who was their poster figure. However, their boss starts going through records and realizing the exact same ten-year old has been used in every poster for the last decade. It turns out she's the real mastermind of all this, in her 30's but with a genetic disorder that makes her look like a kid. The spy chastises her for abusing the trust of people as she drags her out of the wheelchair...only to discover the woman really is paralyzed.
Stargate SG-1: While this show 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.
In the 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. This time, she actually spent those years growing up in a wilderness, and the O'Briens realise that the non-verbal wild-woman she has become cannot be undone, so they return her to the wilderness that has become her home.
T'Pol from Star Trek: Enterprise appears to be in her late twenties but, being a Vulcan, is actually much older. An early episode has her be ambiguous about her age, only saying she is the oldest person on the ship. In the third season finale she mentions that she is 65 years old.
Supernatural: Sam and Dean have both aged mentally several decades beyond their physical ages. Dean spent around 40 years in Hell, which means that he's mentally in his 60s. Sam spent an equivalent period in Lucifer's Cage (it's not explicitly stated how long though). This is only briefly mentioned in the series during season 4 and never mentioned or given any indication of significance again.
Tokumei Sentai Go-Busters: Masato Jin, 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.
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".
Warehouse13: Mrs Frederic, the Caretaker of Warehouse 13 looks fairly elderly as it is but because she's linked to the warehouse she's actually a lot older than she appears. In one episode we met her son who is an elderly old man.
The final scene of the series finale gives a Time Skip "several decades later" and shows us Claudia, who has taken over as Caretaker of Warehouse 13 looking the same age as during the rest of the series, and picking up a few of Mrs. Frederic's tricks
Workaholics: All of the three guys look younger than their age, with them being in their mid-to-late twenties, portraying early-twenties characters. Adam, especially, suffers from this, looking like a teenager at age 26. In the final season, when he was 33, he looked to be in his mid-twenties.
7th Heaven: Beverly Mitchell, who played middle daughter Lucy, was actually one year older than Jessica Biel, who played oldest daughter Mary. Biel was 14 and easily looked 16 or 17, while Mitchell was 15 and looked 10.
Modern Family featured two examples. Sarah Hyland was 18, playing a 15-year-old and looked like she was 12. Adam Devine played a 21-year-old, looked like an 18 year old, and was actually in his 30s.