Middle-aged Detective Kogoro Mouro (Richard Moore in the dub) is an adult watching over Kid Hero Conan. Except he's kind of a Jerkass, and he's not goodat his jobeither. Since he's an adult, people are more willing to listen to him than Conan who uses Kogoru's adult status, solves the crimes for him, and lets him take the credit for it on almost every case. Since he's an on and off ally to Conan it's a Zig-Zagging Trope.
Highschool of the Dead: Shizuka Marikawa is a 27 year old School Nurse, who tends to the other character's wounds, and is the only person of the main characters's group that's out of highschool. She's a skilled Badass Driver, usually in charge of driving the group around, while mowing down copious amounts of "Not Zombies" in the process.
K-On!: Sawako Yamanaka is the teacher in charge of their high school's light music club. Fitting in that she was a rocker in the past too. She's more for comic relief though, and she doesn't use her authority as a teacher very often. Naturally the rest of the characters are high school students.
Naruto has this institutionalized in that their school's ninja teams begin as three twelve-year-olds with one adult teacher assigned to guide them.
Pokémon has one adult that helps out almost every episode. Sometimes it's Officer Jenny or Nurse Joy, but there's a whole list of others they come across depending on the episode. The main characters never age past their teen years.
Films — Live-Action
Bushwhacked is a comedy starring Daniel Stern as Max Grabelski, a sleazy delivery guy who thinks he's been framed for murder. He impersonates a Scout leader with a group of kids while on the run into the woods, and intends to ditch the group later, but they all grow to like each other, and he starts to become the role.
The Rocker: Rainn Wilson plays failed drummer "Robert Fishman" who joins a band of people 20 years younger.
The Outsiders: Everyone in the gang is a teenager, mostly 16-17 with one Tagalong Kid at 14. The one outlier is Darry, who is 20 and the leader.
Wedge Antilles in the X-Wing Series novels, particularly the Wraith Squadron trilogy, isn't chronologically that much older than his pilots — but he's vastly more experienced and way more emotionally stable than the Dysfunction Junction that is Wraith Squadron (to say nothing of his Bunny Ears XO, Janson). He directly compares the squadron to his children several times, and his much-put-upon lament that "Wes, they're doing it to me again... becomes a notable running gag.
Wedge: I'm leading children, and I'm getting them killed.
Janson: That's true.
Wedge: What did you say?
Janson: It's true. (shrugs) Wedge, you asked for misfits. You had to know that even with the ones that made the grade, they were going to take losses that were heavier than in a normal unit. So many of them are carrying around these weights of emotional problems. It makes it tougher for them to hop in the right direction at the right time.
Giles in Buffy the Vampire Slayer, is an adult leader working for the Watchers, who watch over the Slayers. He acts as a father figure to Buffy and mentor to their team, sometimes, but is usually not part of the action himself. This counts before the main characters become competent adults themselves.
iCarly: Spencer often gets involved in the teenage Power Trio's antics, despite being a good ten-fifteen years older than them.
Psych: Harry (a.k.a. "Harold") Spencer, is hired by the Chief of Police to be Shawn and Gus's adult in later seasons. Because they're immature and use questionable methods of solving crimes that get them into trouble, hiring Harry minimizes risk. He was already Shawn's mentor, and his Dad, so it works out.
Square Pegs: In two episodes Johnny's band "Open 24 Hours" performs. It includes himself on lead vocal, some other teens as backup singers/guitar/etc., and John Densmore from The Doors on drums (age about 38).
Whenever Atticus has successfully joined the gang in Todd and the Book of Pure Evil he tries to fill this role. His complete lack of maturity makes him a poor fit for it, though.
Victorious has a few episodes where Sikowitz, the gang's acting teacher, will join in with the group's daily adventure.
The Rolling Stones: Their former bassist Bill Wyman is 7-10 years older than the rest of the band. After he "retired" they didn't add another bassist.
Duck Dynasty: Uncle Si is the only person his age (in his 60's) working at the duck call business. He's a Bunny-Ears Lawyer, who frequently takes naps, snack, hunting, and basketball "breaks", and plenty of other hijinks with the younger guys at work. As much as he'd like to, since no one else will take his job, Willie just can't get rid of him.
Atelier Rorona: Most of the playable characters are in their teens. Then there's the 28-year old Sterk. And later we also get Cool Old Guy Gio, who's in his forties. Both of them are considered the most powerful party members.
The game has the protagonist's uncle's partner, Adachi, who isn't in on the secret (as the police were in 3), but works as a Friend on the Force for the investigation team, who are all high school students. Until he turns out to be the one behind the murders. Said uncle ends up serving the role (again, without finding out about the supernatural aspects).
Igor, the old man who serves as the proprietor of the velvet room, and appears in every persona game to date, to contract with the protagonist, and aid him via persona fusion to obtain more powerful personas. He also serves as Mr. Exposition, to explain the mechanics of personas. He's essentially the Big Good of the persona series, as he's the servant of a Bigger Good Philemon. He's the, first, and most important ally the protagonist has.
Skies of Arcadia: Drachma (pictured above). A bitter old, sailor, who lost his family tothe archwhale, saves Vyse and company when their ship capsizes. Although he's not very happy to accept help from a bunch of youngsters (all three are 17), he does reluctantly agree to let them stay on his ship as long as they pull their weight, and they get sailing experience in exchange. That is until they get separated. Then Gilder replaces him for most of the game. However Drachma's notable because his story arc is developed a lot more than Gilder's.
Jade Curtiss of Tales of the Abyss, who is in his 30's. All his teammates are no older than 18, the only exception being Guy (age 21) who is one of the last members to join after Jade and Anise. Guy doesn't take up much of a leading role either preferring to crack jokeswith (and at) Luke and company instead.
Claus F. Lester from Tales of Phantasia is the only adult in a team where nobody else is older than 18.
Wild ARMs 1: Jack Van Burace. A 27 year old treasure hunter, and expert swordsman, amidst a team of teenagers, trying to save a dying world. His main reason for tagging along is he hopes to find an item called the "guardian blade" while traveling with them so that he can become powerful.
Jack's Wind Mouse companion, Hanpan, might be older than anyone on the team, but the game never explains what a "Wind Mouse" is in the first place.
Wild ARMs 2: Brad Evans is 31 years old, and a war veteran from a revolution years before the story starts. The only other playable character in the party that's technically older than he is is the Immortal,Vampire, Marivel Armitage, but she's an optional character. Kanon, seems to be around the same age as Ashley, but we never learn her actual age. The rest of the party are known to be over a decade younger than Brad.
Higurashi: When They Cry: Detective Akasaka plays this role to the teenage main cast by returning to Hinamizawa on a hunch and ending up protecting them from Takano andthe Yamainu who murder many people, and are out to get Rika too. Ooishi would have been this but he is too aloof and often antagonizes the kids.
Zachary in Bittersweet Candy Bowl during their vacation arc. He's in his early 20's, while the other characters are in their freshman to junior years in high school. He helps the group out multiple times, in a crisis.
Characters in Cucumber Quest have intentionally vague ages. But third member to join, Sir Carrot, is confirmed to be an Adult by Gigi. While "Cucumber and the rest are just kids". Carrot is also a Knight, that's vowed to rescue his Princess Parfait, who is trapped in Caketown Castle. Cucumber's quest to find the dreamsword, and defeat the Nightmare Knight just happens coincide with Carrot's goal.
Paranatural has Mr. Spender as a club adviser, assigned to supervise their club. The rest of the activity club being about 12.
In Adventure Time Jake the Dog, is the closest thing to this in the show, being in a serious relationship, which makes him more of an adult than Finn is, and being old enough to give Finn advice about growing up. Being a dog his age goes by dog years so he matures faster than Finn does. Them being technically adoptive brothers also gives Finn a reason to trust him with a lot of things.
American Dad!: Parodied in the episode "American Dream Factory" where Steve forms a band consisting of his school friends and a coke-snorting Jaded Washout for their drummer. He is eventually replaced by the similarly older Roger.
Barry: We told you, no drugs! Gerry: And I told you if I was responsible, I wouldn't be in a band with a bunch of 14-year-olds!
Arthur: Francine's band "U Stink" consists of her and several of her 3rd grade friends, plus 60-something Mrs. MacGrady on keyboard.
Bravest Warriors has the Emotion Lord as the Token Adult often tagging along with a group of teenagers. He wants to help and be part of the team, but Chris keeps telling him to go away because he is too old. Although the Emotion Lord is actually Chris from the future.
The Wild Thornberrys. In an episode called "The Dragon and the Professor", Eliza Thornberry is dying to see a komodo dragon, but her plans are halted by her tutor who wants her to do homework. The solution? She does it anyway, and her tutor becomes her adult as he tags along with her to find it.
In Wakfu Ruel Stroud, an elder to the rest of the team, joins because he's asked to watch over Yugo by his old friend Alibert who is Yugo's adoptive father. He agrees since he's become fond of Yugo. All of the other characters are younger than 19. Yugo is 12.