A character angsting about how they're getting old is a great source of drama. But how do we kick off this drama? What are some cultural signs of getting old? A milestone birthday? Nope. How would we iron out the birthday business to get to the development of the character's angst? Besides, we might ruin the appeal of the character's Vague Age. Bodily malfunctions like creaky joints? That might imply too old an age. Gray hair? People of all ages get those. But it's mainly associated with middle age. And it doesn't have any lasting impact, as long as they're removed or dyed over. And if it's only one strand, it's not immediately visible to the naked eye. Besides, it's just hair color and doesn't really impact anything else, other than the angst and freaking out about getting old that it is often the source of. Hence we have the First Gray Hair, which automatically equals age. A character is looking in the mirror (or at their hair itself, if it's long enough for that) and suddenly sees a glint of something different from their usual hair color. (S)He gives in to curiosity and inspects more closely to find that it's...GRAY HAIR! And on their own head, too! And thus the angsting about getting old begins.
Some may wonder whether it is worth it to have a character making a big deal over finding that one or more strands of his hair have changed color (even if it is to gray or white). Then again, Tropes Are Tools, and the First Gray Hair is often used for An Aesop about how growing older is a natural process and not necessarily a bad thing once the character has overcome his angst on his own or by force. It's also a harbinger: while the First Gray Hair is harmless by itself, there are more to come, one by one, until it's all gray, or even gone; also, it's tangible proof that they're leaving youth behind. Unfortunately, though, this way of dealing with having found the First Gray Hair seems to be limited to men who find them. Women who have found this tend to have more prolonged angst and are additionally shown removing or dyeing over their grays.
Compare Ma'am Shock. May lead to Midlife Crisis Car if played for laughs. For certain men, however, it may just be the first step on the road to attractiveness.
Old Clairol ad: "Does she or doesn't she? Only her hairdresser knows for sure!" The implication is that if she is (using a Clairol product), it's to cover up the grey and, of course, that it's just the right product to start using once a woman has had this happen to her.
Grecian Formula had commercials for men's version and women's version to help them cover up that grey. Their ads usually started with a man looking in a mirror and visibly unhappy about having this, and the woman being mistaken for her husband's mother rather than his wife. They're happier once they use the product and their grays have vanished.
One Monica's Gang story involved Jimmy Five's father thinking of himself as old and weak after one of his five hairs apparently goes gray. It was actually covered in flour; the story was about his birthday.
At the start of Vanilla Sky, David finds a grey hair and promptly yanks it out. It seems like an innocuous detail, but actually ties into the film's underlying theme.
As quoted above, Willy Wonka tells Charlie that this happening to him is the reason why he started the Golden Ticket contest to find an heir in the 2005 film version of Charlie and the Chocolate Factory.
Disney's Sing Along Songs had one song about the stages of life, one of which involved the man finding his first gray hair and snipping it off with a sour look.
Clarissa Stump has a moment like this in Murder on the Leviathan, and it is implied to not be the first time. She rips it out but is immediately ashamed of herself being in denial about her age.
This is what kicks off the plot in Which Witch?? by Eva Ibbotson. Arriman the Awful has been waiting around for his prophesied successor to show up; now that this one gray hair has shown up, he can't wait anymore, and his manservant suggests that the prophecy means his son. Of course, he isn't married, so this requires him to find a wife...
Played with in The Catcher in the Rye. Sixteen-year-old Holden acknowledges having a great deal of grey hair, but does not seem concerned by it. Nevertheless, it is listed as being one of his 'adult-qualities', which is significant considering the themes of the novel...
Amelia Peabody started discovering her first gray hair, over and over, round about World War One. Every time she spots one, she dyes it from a little bottle that she keeps dead secret (or did until her brother-in-law needed to make an emergency disguise).
A lone gray hair on Alobar's head kicked off the entire plot of Tom Robbins' Jitterbug Perfume, due to his village's rules that they must have a young and strong king.
In his book Time Flies, Bill Cosby said that he did not mind the hair on his head turning gray, because he thought it made him look more dignified. His first gray pubic hair, on the other hand...
While working a spell that requires three hairs in White as Snow, Arpazia vaguely notices one of them is gray. Subverted in that Arpazia doesn't really have any sense of how old she is anymore, and is so disconnected from reality that she believes she's still a young girl.
Live Action TV
In Doctor Who, Captain Jack Harkness is immortal and apparently unaging, but in "Last of the Time Lords" he mentions having found a grey hair, and theorizes that he is aging, very very slowly.
Dr Cox of Scrubs goes a little nuts when he finds a gray hair in his happy trail.
Likewise Samantha in Sex and the City, leading to a horribly botched dye job. At the end of the episode, she solves the problem by shaving it all off.
John Crichton discovers one in the Farscape episode "A Human Reaction". Not that the rest of the episode actually grants him the time to ponder the significance.
In the final episode of LOST, Richard finds one. Though rather than being a source of angst, he considers this a very good thing since it means he's no longer immortal.
One episode of Clarissa Explains It All sees Clarissa's mother panicking about getting older after discovering that she has a gray hair. Clarissa's dad, however, is quick to point out to her that she only had a gray hair, having just plucked it out before both characters entered the scene.
Spoofed in Smallville. The Wicked Witch, Isobel, has taken over Lana's body and needs the hair of a virgin for a potion she's brewing. When Lois Lane has her back turned, Isobel plucks one of Lois's hairs out, but frowns upon realizing that Lois isn't a virgin. Lois, who is unaware that Lana is being possessed by Isobel, angrily asks Isobel why she would pluck one of her hairs out. Isobel lies and says "It was gray," to which Lois insists "I don't have any gray hair." Isobel shrugs and looks innocent. Lois gets a nervous expression on her face and says "I'll...be in the bathroom using your mirror," and hurries off.
The episode "One" in the 6th season of Sex and the City wholly revolves around this theme. Samantha (actress Kim Cattrall) finds one gray hair. The twist is, it's a pubic hair.
Mostly averted on Going Gray Looking Great, though some members on the site share stories about experiences fitting this trope.
Thoroughly averted on the Salt & Pepper Thread on the Long Hair Community forum in that those who regularly post in that thread don't mind and even celebrate what they feel is the beauty of their gray hair. Played straight in the accounts members share about feeling old upon finding their first gray hair(s), but subverted in that posts about finding gray in one's hair is considered a cause for celebration.
One episode of Totally Spies! had Sam finding a gray hair in her head (unusual, considering she's a teenager, but not thoroughly impossible) and she spent much of the rest of the episode worrying about her age and seeming young. The Aesop of the episode, as delivered by a Cool Old Lady: "It's not how old you are, it's how you are being old!"
The Simpsons: this happens to Marge Simpson when she finds one gray hair in her blue beehive.
Alvin and the Chipmunks once painted a small portion of Dave's hair white to give them an excuse to treat him like he was treating his Uncle Willie (a.k.a. Uncle Adventure). Namely, treating him like he couldn't do anything by himself.