Eternally Pearly-White Teeth

"Boys, would you take a look-see at these pearly whites! Hell, I ain't seen teeth that straight that warn't store-bought."

—Buford Tannen‍'‍s henchman, Back to the Future Part III

Characters in nearly all media are portrayed as having pretty, white teeth, with nary a hint of any kind of tooth decay whatsoever. This makes plenty of sense in the right context—TV personalities, for example, are heavily made-up, so of course their teeth are going to look pretty. The same goes for anyone thoroughly obsessed with personal vanity. Even stretching plausible deniability, there are perfectly normal people in the world who have never had a cavity. Expect variance in the shade of white: American media tends to go for striking white, whereas in some other places the look is considered "Yankee-ish" or even unappealing, and teeth depicted will be a more modest shade of pearly.

At the other end of this trope is when people who really should have terrible teeth don't. In fiction, teeth never wear down through extended using of Mundane Utility. They never seem to be particularly burdened by long periods of time spent in barren environments without dental supplies. And even when the apocalypse hits, somehow, those teeth always stay nice and clean.

This trope is so common that it's simply easier to list aversions, subversions and Egregious examples. It's just that common.

Advertising

Crest 3-D White Toothpaste shows a woman promising to introduce her friend to someone - in two weeks, after she's whitened her teeth. As she says the last bit, her friend's smile, which was exposing her incredibly white, perfect teeth, falters. At the end of the commercial after the treatment she smiles again, showing teeth that look identical to how they were before. Maybe the difference is apparent to some people, but one would think that if there were one place on television to see slightly less than perfect but not joke-level teeth, its in the "Before" section of a tooth-whitening ad..

Film

Averted in Back to The Future Part III, where Marty's unusually clean teeth are one of the first things that people notice about him. Doc's teeth, however, are not commented on (Well, they've had more time to get used to him. Also, he grew up in an earlier generation of dentistry than Marty).
10,000 B.C.: Close-up shots made this glaringly obvious.
Averted in Shanghai Knights: Owen Wilson's character smiles at a pretty girl, then recoils in horror when she smiles back and reveals a row of awful teeth.
Pirates of the Caribbean averts this not just with the regular background pirates (who are the sort of people you'd expect to have bad teeth), but also with Jack Sparrow. Played straight with Will Turner, though..
Monty Python and the Holy Grail: Hilariously averted, if not outright inverted. The Pythons noted on the DVD audio commentary that real medieval English peasants probably had nicer teeth than those in the movie, due to the lack of refined sugar and other tooth-rotting foods in The Middle Ages.
Gangs of New York: Several reviewers noted that the main character has crooked teeth as a child, but perfectly straight teeth as an adult. Dental braces were invented about half a century after the film's setting.
New Moon: The makeup artists for went a little nuts on Taylor Lautner's teeth, enough for a hilarious reaction from the boys:

"Whoa! You could land planes with those teeth! Ease up on the Crest strips, will ya?"

Twenty Eight Weeks Later: The mother has been living in squalor for months. Her room is a dirty mess. She's a dirty mess. How nice that it looks like she's kept her teeth neatly brushed and flossed.
Averted in the French film Les Visiteurs as people from the Middles Ages actually have very dirty teeths. Played straight in its American remake, Just Visiting.
Both played straight and averted in The Lord of the Rings - the Orcs get absolutely horrendous gnashers, as the makeup people point out that good teeth are a sign of 20'21st century living - but, as noted above in the picture of Boromir, all the good characters have perfect whites.

Literature

Animorphs: An aversion; they hunt Visser Four throughout history—in most of the time periods they visit, the Visser's host's cleanliness (teeth included) makes him stand out.
Harry Potter:
- Averted where Severus Snape is described as having yellow teeth. Not that fanfic writers seem to have noticed...
- Sirius Black, ditto on both the yellow teeth and the fangirls.
Frankenstein's Monster had "pearly white teeth", because Frankenstein deliberately built him out of attractive parts in hopes of creating as perfect a specimen as possible, but instead the overall effect is Uncanny Valley.
Peter Pan: Peter also had pearly white teeth as it is commented by someone in the book, and they were still his baby teeth too.
In Harry Turtledove's Case of the Toxic Spelldump, the main character noted that the foreman had his teeth unbelievably white, due to sympathy magic connecting its whiteness to the pure snow of the alps. Then he muses what would happen to the guy (and several celebrities) if a forest were to burn down and cover the snow with soot....
The Warlord Chronicles: Arthur and Merlin are the only ones with perfect teeth and this is commented on as unusual; Arthur because he actively takes care of them, Merlin just by happenstance, though he loves being smug about them.

Live Action TV

Top Gear: Teasing Richard Hammond about this is a Running Gag, however much he says "I have not had my teeth whitened!"
Parodied in Friends, when Ross' teeth whitening treatment reacts to a blacklight.
The John Adams miniseries tries to avert this by coloring and blacking out people's teeth, but it's inconsistent. When Abigail Adams is on her deathbed, it looks like she has had some late-life dental work done.

Video Games

Civilization III: Inverted in the leader pics. Genghis Khan has hideously bad teeth—even in the modern era, when logically even the Mongols ought to have reasonable dental hygeine.
Averted in Dragon Age—every single character in the game (including your own), be they a human, an elf, or a dwarf, has stained yellow-brown teeth. Although the game takes place in a fantasy universe, this is very appropriate considering it parallels medieval Europe. Played completely straight in the sequel. Everyone has perfect white teeth.
M. Bison, the Big Bad of Street Fighter. When he appears in modern games with better graphics, always has a gleaming smile; clearly, even villains in Fighting Games know the benefits of never forgetting to brush.
Wario has never had a cavity in his life! Says so on this chart included with the instruction booklet for Wario World.

Web Original

The narrator of this video, detailing the Atomies (a Fey race in Dungeons & Dragons) claims he once asked the little fairies how they managed to maintain such perfect teeth despite frequently eating honey and nectar while having no access to modern dental hygiene. They told him to try chewing tree gum, though he figures they were trying to mess with his head.

Western Animation

Codename: Kids Next Door: An Egregious example is in an early episode, when the kids are showing Numbuh Three what kind of teeth she should have, they are revealed to have hideous teeth due to poor dental care. And yet, in every other shot, they have perfect teeth. Recurring villain Stickybeard averts this trope with hideous teeth due to all the candy he eats.

Truth In Television

Caries is caused by sugar-eating bacteria, therefore people who have a diet poor in carbohydrates do have good-looking teeth even if they never clean them. This is also true for cavemen: Contrary to popular belief, people before the development of agriculture (and the diets heavily based on carbohydrate-rich cereals that were brought by it) did rarely have a dental condition. The neanderthals in particular, which were mostly carnivorous and so rarely ate carbohydrates (to the point their bones show levels of nitrogen comparable to those of wolves) had perfect teeth until the pieces began to fall when they were around 40 because of a lack of vitamin C in their diet - which affects the periodontium, but not the enamel.
Cavities are caused by acids that are the byproduct of bacteria breaking down sugars in the mouth, but the loss and staining of teeth is caused by other bacteria which affect the gums. So you'll have strong teeth, its just that they will be slightly yellow and fall out sooner compared to today.
- There is also archaeological evidence that, despite not having access to flouirde-fortified minty goodness, pre-modern humans did indeed take measures to clean and care for their teeth through chew sticks and other methods, so dental care in the past may have been better than is often credited.
This may be unintentionally enforced by actors, since brushing one's teeth is a long-ingrained part of the daily routine. Plus, it's most likely a case of Selective Squick. If you gain/lose weight for a role, you're incredibly dedicated, but if you don't brush your teeth? That's just gross.
- Actually even with brushing every day and dental most people naturally have slightly discoloured or of kilter teeth and smiles like these are often the product of treatments that didn't exist back the or weren't very effective. Case in point British comedians v American actors- most Brits do brush their teeth but tend to scorn people who bleach or whiten them (e.g. in Top Gear that's one of the things Richard Hammond is mocked for) whereas in America that would be seen as neglect.
There are people who have a born immunity to dental bacteria, and are almost unable to develop cavities until very late age. There is actually a project for developing a vaccine to emulate the effect, but the progress is slow due to low funding; the dentists naturally aren't very excited about the idea.