Simply put, this is the belief that old people don't have sex.
There is a longstanding idea that growing old stops sexuality dead in its tracks. This could be due to the notion that sex is too tiring for frail elderly bodies and the "equipment" no longer works. Likewise, this idea may be caused by the idea that sex is only for the young, beautiful, and fertile. Or maybe it comes from the wholesome ideal of grandparents or the idea that adults must uphold decorum and propriety past a certain age. It could also be from the stereotype that long-lasting couples eventually suffer from Dead Sparks. Of course, it could simply come from the fact that many adult writers don't want to think of their parents' generation having sex.
In fiction, this stigma manifests as elderly characters being largely sterile; even DecemberDecember Romance works, which focus on elderly romance, may skimp on the hot and heavy bits of a relationship. Sexually active elderly characters will be treated as non-existent In-Universe, considered shocking or gross, or the butt of jokes. Elderly characters are more likely to wax poetic about when they were young and sexually attractive, implying that they aren't anymore.
This belief used to be prominent in LGBT+ circles. The focal characters of Queer Media would skew younger and no elderly LGBT people would be shown. This is becoming less prominent since more people of various ages have been coming out and the original population of "younger" LGBT+ characters are getting older.
This is part of why the Dirty Old Man and the Dirty Old Woman are considered off-putting or funny — not only do they unashamedly hit on characters much younger, but they're old while doing so. However, the Cool Old Guy and the Cool Old Lady will often be more liberal and "hip" about sexual attitudes, and will often imply they were sexual in their youth, whether or not they are now. This may also be a reason for Parental Sexuality Squick, once one gets the parent part out of the way.
In Real Life, this notion is becoming more and more discredited, as active sexuality is still considered a normal part of aging.
In-Universe Examples Only: Characters must express the idea that old people don't engage in intimate acts, or shock/disgust that they do, regardless of whether or not this is true for the work's elderly characters. Alternatively, an older person may outright say they haven't been having sex past a certain age, or their sex life can be played for laughs or faux horror.
- In Gods and Monsters, James Whale is an elderly, promiscuous gay man, and his gardener, whom he's sexually provoked, says Whale is too old to think about things like sex.
- The Object of My Affection: Rodney is older, gay, and explicitly currently celibate, though it is never explained if this is a personal choice or a physiological problem.
- Discussed in X, when Wayne first reassures the apprehensive Maxine that the elderly Howard's "pecker" probably hasn't gone up since before she was even born and later when Bobby-Lynne remarks that everyone eventually grows too old to screw anymore and life's too short to let that happen.
- Captain Underpants and the Plight of the Purple Porta-Potty People: George and Harold are both horrified by their grandparents' Big Damn Kiss.
- Downplayed in Dirty Bertie, which, being a kids' series, doesn't mention sex but it does mention Bertie being really grossed out by the idea of his grandmother kissing her boyfriend because she's old.
- The Stupidest Angel A Heartwarming Tale Of Christmas Terror includes a disclaimer in the front warning the reader that the book contains graphic language, violence, and "people over the age of forty having sex".
- The Accidental Further Adventures Of The Hundred Year Old Man (sequel to The Hundred-Year-Old Man Who Climbed Out the Window and Disappeared): subverted. The 101-year-old protagonist and the 50-ish Sabina spend the whole night awake and active. The 50-year age gap is Played for Laughs.
- In the US version of Getting On, the nurses scramble for what to do when they realize that two of the patients in their geriatric extended-care wing have been getting it on, something that has apparently not happened before. It is resolved through, of all things, paperwork; Dawn has to have the old woman's grown sons sign a consent form.
- How I Met Your Mother: In one episode, Ted and Robin are squicked out about the couple in the apartment above them loudly 'bagpiping' (Future Ted used that as a placeholder in one of the rare times he held back in telling his kids something). When Ted finally has enough and goes to tell him off only to back down when he realizes they're elderly because 'good for them'. In a later episode, Robin and Barney accidentally catch her grandparents doing the deed.
- Metrosexuality: Discussed. Max states many times that growing old is basically forbidden by queer culture.
- In the Parks and Recreation episode "Sex Education", Leslie holds a meeting for seniors to discuss safe sex practices. At some point, Andy suddenly realizes that old people do have sex, at which point he is extremely grossed out.
- Porridge: At one point Fletcher tells a joke about an elderly man who complains to his doctor that he and his wife aren't getting the enjoyment they used to from sex. "When did you notice this?" the doctor asks. "Twice last night and again this morning," the man replies.
- Roseanne: Bev gets kicked out of the retirement home because she was having too much sex, much to the disgust of her daughters Roseanne and Jackie.
- Headmaster Groff in Sex Education believes this. When his wife attempts to initiate sex after a years-long dry spell, he scoffs and dismisses it with "We're not 23 anymore, Maureen." Sex therapist Jean defies this notion, as many of her clients are older, and she gives Maureen advice on the topic.
- Sex and the City: Samantha, a woman in her forties dating a man in his twenties, alludes to this mentality when she starts finding gray hairs, and worries that he'll stop seeing her as older and start seeing her as old.
- Schmigadoon!: As part of the town's outdated sexuality and gender politics, Doc Lopez disallows his elderly parents from borrowing lube from him, saying they are "too old for the act of love".
- Scrubs: A few people from a nursing home have been admitted to the hospital with a mysterious illness. When Elliot learns that it's an STD that's been spreading around the nursing home, she is grossed out.
- Sirens (US): In one episode, the guys try to get the image of a porn film involving a woman and a horse out of their heads. Then they burst in on two elderly people in the throes of passion, which grosses them out so much that they nearly accept the woman and the horse.
- Heavily averted in The Lonely Island song "Boom Box," where turning up the turbo bass on the titular boom box in a nursing home inspires its residents to immediately engage in an orgy.
- Jeff Dunham's puppet Walter was pretty shocked to learn that Jeff's grandparents were still sexually active well into their 80s, seeming to be a subscriber to this trope. This could be something to do with things not getting too exciting under the sheets for Walter himself, being an old man as well. It's not helped that he's not too happy with his own marriage.
Walter (as Jeff's grandparents): "Was it good for you?"
"I don't remember..."
"But we just finished!"
"Who are you?"
- The Non-Adventures of Wonderella: In this strip, Wonderella is helping an elderly couple with the young Bedsheet Ghost haunting their home. When the old woman complains that the ghost's presence interferes with sex with her husband, the ghost is so disgusted that he ascends right to the afterlife.
Wonderella: Ghosts can't really vomit in their own mouths. He was out of options.
- In the xkcd strip: "Ages", people 55 and up are marked with "more sex than anyone is comfortable admitting".
- On this video called "Things Not to Say to an Old Person", one of them is "Sex?! At your age?!".
- The Bob's Burgers episode "It Snakes A Village". Linda is disgusted upon realizing her parents live in an elderly swinger community, while her husband Bob finds it funny.
- Futurama: Professor Farnsworth and Mom are both over a hundred years old, and any time they bring up their former sexual relationship, everybody gets squicked out. And then there's the time the crew walked in on them getting intimate...
Fry: Nothing in here but a couple of elephant skin rugs. [realizes what's actually happening, and gasps in shock]
- The Simpsons: Homer holds the idea that his elderly father Abe leads a celibate lifestyle. When Abe tries to help Homer's sex life with Marge, he gets horribly disgusted over the subject.