Christmas Cakes sometimes get a craving for a taste of the younger kind (no, not thatyoung - well, not usually...).
Mrs. Robinson is usually much older than her lover, and her lover may or may not be willing. Even if the younger party is unwilling, not all people are sympathetic to their plight. In some cases, the boy is just some Unlucky Everydude, feeling isolated and looking for love. In more comedic situations, a Casanova Wannabe or Chick Magnet may attract the attention of a much, much older woman - which he desperately does not want!
With the genders reversed, you almost always end up with a Dirty Old Man on one side and an innocent girl on the other. Since Mrs. Robinson is sometimes portrayed sympathetically, this may be an example of a Double Standard.
Do note that whereas the Absurdly Youthful Mother retains enough of her youthful beauty and vibrancy to be often mistaken for a girl of her children's generations, the Mrs. Robinson is a woman clearly and visibly in the twilight of her girlhood glory, making her need for validation even more sympathetic to the audience.
Unlike a Mayfly-December Romance, it's not the actual age which is problematic, but rather the apparent age that causes so much Squick.
This is an Evolving Trope because society is gradually becoming more accepting of the sex appeal of women aged 30+. Some now refer to older women who still have active sex lives as "the cougar" and what they are interested in as "cougar sex." This also led up to the coining of the term "MILF" - a subversion in which people who like older women actively wish to be seduced by them. Sometimes they are the ones themselves who seduce older women. If every younger character around her finds themselves tempted to do this, she might be a Stacy's Mom. There are also the "pumas" who are in their late 20s to mid 30s who sleep with younger men.
Compare to Hot for Student.
Fille Fatale and Likes Older Women are inversions. Contrast Dirty Old Woman, who's even older and Played for Laughs.
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Anime and Manga
Allegedly 19 year old Haruko from FLCL borders on Shotacon when she flirts with 12-year old Naota: the secret agent who talks to Naota even says "An older woman? That's a big mistake... well, she is very alluring. It's not your fault; you're just her current victim." He would know, since it's later revealed that Haruko used him too.
Gantz Sei Sakuraoka is one. Kei Kurono is 15, and she appears to be anywhere from 5 to 15 years older than him. She approaches him, flirts with him, and sleeps with him...twice.
Ghost in the Shell: Stand Alone Complex somewhat uses this in an episode in which the Major (who if she were more biological would be in her 30s) meets a teenage (14-15) drug dealer in Taiwan by way of rescuing him from a group of thugs. When she gets a room for both of them in an upscale hotel, the receptionists assumptions are quite clear. When asked about the sexual capabilities of a cybernetic body, her offer to show him seems pretty genuine, but the boy declines; thereby spawning about a half dozen H-Doujins where he takes her up on that offer.
In Billion Girl, there's a case of this... which is disturbing, since it's between lead male Taiga and his aunt, and it's more or less implied to be rape. Made even more disturbing by the fact that he resembles her son to where they look like twins.
Kane Ohtori's mother Mrs. Ohtori in Revolutionary Girl Utena is almost a textbook case of this, and like most things in Utena is intended as a deconstruction, specifically of the Fallen Princess trope. Mrs. Ohtori's husband is ill, so she tries to find validation in the worst possible way, mainly sleeping with Akio, the Big Bad, who's also her daughter's fiance, something Akio enjoys reminding her of as they're about to do it.
In Stardust Wink, out of emotional security, art teacher Mari Kobayashi latches onto her 15-year-old student, Hinata, who reluctantly lets the casual relationship happen to displace his feelings for Anna, on whom his best friend Sou also has a crush. The farthest they've ever gone was a peck on the lips, though.
Played horribly straight by Nami's mother in Kasei Yakyoku. She attempts to rape an almost cathatonic Taka who's in his mid-to-late teens tops... and her husband shows up and kills her right there.
Pepper Poptart, Cherry's mom from Cherry Comics, who has a habit of seducing her 18 year old daughter's boyfriends.
In the Silver Age and Bronze Age of Comics, Black Canary was this to a younger Green Arrow. A later story retconned things so that she was actually the daughter of the original Black Canary, with her mother's memories magically implanted into her, so as to explain why a superheroine who should have been in her 60s didn't look a day over 30. She also served in this role, after Green Arrow's death, to a younger hero - The Ray.
In A Contract With God by Will Eisner, one of the characters, who's a teenager, loses his virginity to a beautiful older woman. Her husband shows up and then proceeds to have sex with her in front of the adolescent boy.
The Trope Namer is Mrs. Robinson of The Graduate, a married mother of one who seduces recent college graduate Benjamin Braddock, and engages in a lengthy affair. Later, Benjamin falls in love with Mrs. Robinson's daughter, Elaine - who later discovers that Benjamin had been sleeping with her mother. Awwwwwkward!
Evolving Trope: the MILF note Mother I'd Like To Fuck phenomenon started a subversion of the initial trope. The American Pie movies introduced the word MILF to mainstream culture when (the initially highschool graduate) Paul Finch seduced his friend Stifler's mom. While she doesn't pursue her relationship with Paul, she is rather promiscuous towards others. In American Pie 3: American Wedding, Steve Stifler accidentally slept in the dark with his friend Jim's grandmother (though he surprised her, she seemed to have enjoyed the experience).
An extreme example would be the movie Harold and Maude. People get very upset over a romance which is quite beautiful, even if Harold is maybe 18 and Maude is 80.
In Breakfast at Tiffany's, Patricia Neal takes an interest in George Peppard's ... writing. Although the actors' age difference is only two years, it is implied she is sufficiently older than he to evoke this trope.
The film Summer of '42 implies that these sorts of relationships weren't all that uncommon while the grown men were all off fighting World War II. May or may not have inspired the similarly themed song "War is Hell (On The Homefront, Too)".
In My Tutor, 17 year old Bobby loses his virginity to a puma, his 29 year old French tutor Terry
Miranda Richardson's portrayal of Rita Skeeter in the film version of Harry Potter and the Goblet of Fire comes off as this to some viewers. Whereas in the book, she was merely a self-serving bitch, here she appears more interested in Harry than any 30+ year old woman should show in a 14-year-old boy. She had an unhealthy interest in him in the book as well, just possibly not for the same reason.
Edward Scissorhands is only barely saved from the clutches of Joyce, who could be maybe 20 or 30 years older than him. Who also might technically be younger than him, since he is not apparently capable of aging, but it hardly counts, with his childlike mentality and looking like a young Johnny Depp.
In The Secret Of My Success, the company president's bored wife pursues the mail boy protagonist (who is also her nephew, though she didn't know it at first).
The Rebound has Sandy (40) pursuing Aram (25). He's definitely willing though.
In Onibaba, Kichi's mom tries to pull this on her boyfriend, Hachi, because she's afraid he and Kichi will run off together and leave her all alone.
Queen Guinevere from the King Arthur legends is sometimes considerably older that her lover Lancelot. This varies from one version to another even back in the Middle Ages.
In one version, Queen Guinevere is much older than her lover knight Lancelot. She married Arthur some time before he was born. Though considering the age at which noble medieval women were married off, probably not as large a gap as it sounds.
In Lancelot du Lac, the first version to give a full account of their affair (previous Lancelot romances had concentrated on just one main adventure), she is indeed much older than he is. In many later versions, including Le Morte d'Arthur (on which The Once and Future King was mostly based) tell it differently.
In Porterhouse Blue, a college's middle-aged bedder realizes the college's only research graduate student has secret feelings for her. Deciding to act on it, she sneaks into his room in the middle of the night. It ends badly for both of them.
Henry Fielding's Joseph Andrews has the title character pursued by his mistress (he's a servant) Lady Booby. His first name is a Meaningful Name, referencing Joseph of The Bible, who was pursued by Potiphar's Wife.
Mat and Tylin from The Wheel of Time books. Queen Tylin isn't particularly old (though older than 20 year-old Mat), but neither is she receptive to Mat's lack of interest. Since she's got the castle staff on her payroll and is handy with a knife, however, Mat doesn't have much choice. Mat at first panics about her adult son Beslan finding out...until he finds out that he already knows, and approves (his mother was a widow and not interested in remarrying, but taking a lover was perfectly all right). Granted, when Beslan found out that Mat couldn't leave the relationship, he expressed mild disapproval of his mother's actions (but did nothing). It's also an example of Black Comedy Rape.
Literary example in Leslie Marmon Silko's novel Ceremony: The protagonist Tayo loses his virginity to his uncle's mistress. She comes onto him, and the anticipation freaks him out, but he sure enjoys the experience and remembers it fondly.
In Tom Holt's J.W. Wells & Co. books, Rosie, the mother of Mr. Tanner (one of the partners of the J.W. Wells firm) has a thing for young human men, and constantly flirts with the Hapless Everydude character of Paul Carpenter. However, as she is a goblin with the ability to shapeshift into a variety of beautiful young human women, the age and species difference is not apparently obvious unless she reverts to her true form. She gets some of the funniest lines in the books and shows a manipulative streak worthy of a Magnificent Bastard, though she schemes for fun and mischief rather than an end goal. However, she is somewhat sympathetic as she is the only character who looks out for Paul and tries to help him in dangerous situations.
Since Caesar and Brutus are roughly the same age in Conn Iggulden's Emperor series, Caesar's relationship with Brutus' mother Servilia fits this trope.
Literary example: Rule of the Bone, in which the main character loses his virginity to his father's mistress.
The novel And Here's to You features a Mrs. Robinson and her boyfriend (who is half her age, but equal her intelligence) as the central couple. Surprisingly few people even comment on it at all, let alone have a problem with it, the only character in book who seems to think ill of the pairing isn't exactly a reliable moral barometer on account of her cheating on her own husband.
She had masses of dyed red hair and wore short skirts and spangly tops, clearly blissfully aware [sic?] that she was several decades past the suitable age for such attire. She reminded Fitz of the mother of the girl who worked in the greengrocer’s down the road from where he grew up. She’d had a crush on him as well.
In Terry Pratchett's Lords and Ladies, Nanny Ogg is somewhat a case of this with Casanunda, although its not clear on whether or not they... you know. But in doubt, as the Lords and Ladies interrupted their 'romantic dinner.' Then again, given that Casanunda is a dwarf — with a lifespan several times that of a human — it's not clear whether he's younger or older than her.
Invoked and averted in From a Buick 8. Sandy wonders if Fair Cop Shirley has designs on young Ned and that's why she's being so open with him, but in the end nothing comes of it but his speculation.
In Bad Monkeys by Matt Ruff, Jane Charlotte had a habit of sleeping with teenage boys when she was in her thirties. The organization devoted to fighting evil that she works for finds out about this and is not pleased.
In Fifty Shades of Grey, Christian Grey reveals that he got involved with an older woman when he was only fifteen years old. Anastasia is quick to compare her to "Mrs. Robinson".
In Jane Austen's early epistolary novel Lady Susan, Lady Susan is about 36 (not extremely young for Regency England), but is said to look very lovely. She tries to get it on with Reginald, a young man in his early twenties.
In the third Bridget Jones book, Bridget is a widow at 51 and has a relationship with a much younger man she meets on Twitter; causing them both to think of her along these lines. Lampshaded when Bridget has to decide whether to bring him as a date to her friend's sixtieth birthday party, as it turns out his thirtieth birthday is the same day.
Live Action TV
An episode of Supernatural had twenty-something Sam being ogled and groped by a wealthy, lusty woman of 60+ years. He was made intensely uncomfortable by her treatment, though he managed to avoid actually being forced into having sex with her.
In The O.C. Julie Cooper had an affair with her daughters ex-boyfriend
Tanya Turner from Footballers Wives seduced her estranged husbands 17 year old teammate who is considerably younger than her.
In Rules of Engagement, we learn that Russell's first sexual encounter was of this nature. He later becomes obsessed with sleeping with the woman again to prove to her that he has improved as a lover.
Ben and Ruthie in Carnivŕle, although both admit that it was a mistake after the fact. Ruthie, who is at least in her fifties, had been trying to seduce eighteen year old Ben throughout the beginning of the series, until he is conveniently delivered into her bed, naked and in need of comforting, and she sort of takes advantage of his vulnerability.
In Buffy the Vampire Slayer, a praying-mantis monster disguised itself as a Mrs. Robinson in order to lure male virgins for her to eat.
Played with when the young intern-of-the week gets freaked out when Dr. Brennan asks him if he'd consider dating an older woman. Brennan's not actually hitting on him, though. She's trying to get an angle on the current case, which does seem to involve a Mrs. Robinson situation.
In another episode the Victim of the Week was carrying on an affair with a much older woman (the wife of the college President where he was going to school). She is explicitly referred to as a Cougar.
The Duchess of Wharfedale in the TV movie Horatio Hornblower: "The Duchess and the Devil". She tries to seduce Horatio a bit, but it's implied that their relationship is in fact quite innocent.
An episode of Frasier centers on everyone finding out Frasier's first lover had been his much older piano teacher when he was a teen. They meet up in the present, at the end of the episode, only for him to discover she's still dating much, much younger guys, and is no longer interested in his middle-aged self. However, it's important to note that she's not that much older than he is; she just seemed that way to the late high school aged Frasier.
Spencer Shay as part of a sub-plot where he's trying to sneak a sculpture into a museum to impress his grandfather, ends up performing a "Shut Up" Kiss a woman who is obviously older than him due to it being somewhat realistic as she's the curator of the large scale art museum. Once the first kiss is done and his grandfather walks off satisfied, the woman then grabs him and kisses him again.
There are the numerous episodes which suggest a relationship between Spencer and Freddie's mother.
Ross once kissed Mrs. Altman, his 50 year old high school librarian. "She needed help with her word jumble and one thing led to another. If you must know, Anita was very gentle and tender. May she rest in peace."
Ross once made out with Chandler's mother, leading to Chandler uttering the immortal line, "You kissed my best Ross!"
Phoebe's half-brother Frank marries his home-economics teacher, who is 26 years his senior.
Barney sleeps with his mother's friend after Shannon leaves him, which acts as a catalyst for his current personality, which we find out in the episode "The Yips". The woman had a reputation for being a man-maker and she slept around with numerous young men.
In "Aldrin Justice", Barney spends the entire episode "huntin' cougar" in an attempt to seduce Marshall's law professor. She's older but still gorgeous, played by Jane Seymour.
In "The Window", Barney accepted Marshall's non-challenge of getting sex while wearing Marshall's old coveralls. When it looks like he's going to fail at it, the older woman, who was Ted's lookout, takes an interest in him.
Discussed briefly in one episode when Robin, who is not exactly a kid-person, dated a guy with 5-year-old son. She mentions she would like to meet him rather later than sooner, hopefully when she's a cougar and he's as hot as his dad.
This is played with in "Let's Kill Hitler", when a recently regenerated River Song catches sight of the Doctor and saucily says "Well, helloooo, Benjamin!"
In My Name Is Earl, Earl has a drunken one night stand with Ralph's mom. To avoid being killed by Ralph, he ends up marrying her.
Nate Archibald on Gossip Girl has a way of ending up with this type of woman, to the point where Chuck Bass asks for his advice on how to bed one.
In the episode "The Rhodes Not Taken" of Glee introduced us to the character April, an adult. After being asked to try and make the Glee kids more comfortable around her, she is shown exiting the showers with two of the football Glee kids, with everyone smiling.
In an episode of "Who's the Boss", Mona comes home with her boyfriend, who is in his twenties. When she starts to show off the gold broach he gave her, which he pinned over her left shoulder, it's missing, so she thinks she dropped it. As the family looks for it, her grandson finds it, pinned over her right shoulder blade.
In one episode of The Fresh Prince of Bel-Air, Uncle Phil's old friend seduces Will under the guise of interviewing him for her book. What makes things worse is that he had just gotten back from going on a date with her daughter. Also her name was ironically Miss Robinson.
"I've not been swept since the day my husband died!"
"Mrs. Robinson" by Simon & Garfunkel was originally a part of The Graduate soundtrack, before being released as a single. It wasn't originally about this trope, but the director of The Graduate was obsessed with Simon and Garfunkel's music at the time.
K.T. Oslin's "Younger Men."
In the music video of Toby Keith's "You Shouldn't Kiss Me Like This (Unless You Mean It Like That)" a eighteen/nineteen yr old boy has a vivid daydream of he and his older female employer falling in love and making love with one another. The subversion/aversion comes from the fact her real personality is vastly different from his fantasy of her, and she seems to barely recognise his existence.
The Marschallin in Richard Strauss's opera Der Rosenkavalier is a borderline case: she is probably just thirty-something, but being in the same sack with a sixteen-year-old boy makes her lament her age.
Marcellina in The Marriage of Figaro tries to marry the title character, who is young enough to be her son. Until it turns out that he really isher son.
An option, if not the requirement, for several Dating Sims.
Phoenix Wright: Ace Attorney has Wendy Oldbag (she's a really old and short tempered woman who works as a security guard) constantly hitting on The Rival Miles Edgeworth. She even calls him "Edgey-Poo" and sends him flowers, to his annoyance. What's irked up more is that there's a possible indication that she's married (or at least has been) when fans of the series pointed out she has a ring on her finger if you look close enough. During one of her Motor Mouth moments, she mentions that she has a grandson. She also seems to have a thing for Jack Hammer and Juan Corrida, two of the series' victims, who are in their 30's and 20's respectively, though not nearly to the degree that she crushes on Edgey-poo.
In Persona 3, a character believes his teacher is a Mrs. Robinson turns out this is a complete fantasy: said teacher is about to get married with someone her own age, later subverted with the main character HIS teacher admits to him that she would love to play the role of Mrs. Robinson with him: the subvertion comes from the fact that said teacher does not knows who she's talking to: they are communicating via avatars in a MMORPG.
It features a night shift nurse, Sayoko, who playfully flirts with the protagonist in a Mrs. Robinson fashion. However, if you progress her social link enough, she will admit that such behaviour is really a cover for her own feelings of despair, due to losing touch with why she became a nurse in the first place.
The Protagonist's Christmas Cake home-room teacher, Kashiwagi, attempts to invoke this trope on the protagonist and his male friends. They were squicked beyond tears.
In the "Return to Ostagar" DLC for Dragon Age: Origins, Wynne, who's at least in her sixties, flirts shamelessly (bordering on outright propositioning) with 20-something Alistair. He's rather startled. Though she warns him that "when you wake up beside me, I'll be back to reminding you of your grandmother," she also notes approvingly that it wouldn't be the first time she's had a younger man in her bed. He doesn't take her up on the offer (though Zevran certainly would). During the Male Human Noble origin, Lady Landra mentions the time she got heavily inebriated at their mother's Spring Salon and spent the entire evening shamelessly flirting with them... right in front of her husband and son.
Kingdom of Loathing has as a random encounter the "werecougar", described as being "a large mountain cat which, by the light of a full moon, turns into a drunken middle-aged woman with a predilection for younger men".
L.A. Noire has a middle-aged actress/accident victim coming on to Det. Cole Phelps in this manner. He declines.
Sarah and Taylor's mother in lonelygirl15. She's always flirting with - and sleeping with - men in their 20s.
Webcomic example: Polk Out has Polkster wanting to "go all Graduate" on his friend Elana's mother. 
Even though Anya, who is 35, was once Eleanor's love slave in Groovy Kinda, she was the exception. Eleanor much prefers high school boys and girls. Her latest playthings are the Dubbleman Twins, Esther and Jacob.
Played with in Kronk's New Groove, where Yzma is trying to get Kronk's help in selling her fake youth potion.
Yzma: [pushes Kronk back onto a sofa and climbs on top of him] I've been watching you, Kronk. And I have... a proposition for you.
Yzma: ...a business proposition.
Kronk: Oh. Phew!
In the original movie The Emperor's New Groove, we're told that Yzma has a revolving cast of hot young male assistants ("Every decade or so she gets a new one. This year's model is called Kronk."), and it doesn't take much to infer that Yzma is keeping Kronk around for some December-May lovin' (it's all in the delivery):
Kuzco: So. He seems nice.
Yzma: H-he is....
Kuzco: He's, what, in his...late twenties?
Yzma: I'm not sure....
Dee, dee-dee-dee dee dee, dee-dee dee dee, dee-dee dee...