Because we expect an older character to have more experience, aSidekick older than the main hero is a rare thing. Outside traditionalSuperhero comics, there are several reasons to use them.
One is that the older character doesn't fall out of theCompetence Zone because they're under the direction of the hero, and thus doesn't undermine the maturity of the younger hero. If the relationship is close, the sidekick's more adult personality actually fulfills some parental role, which the hero might lack from havingno family, without being intrusive. On the other hand, theOlder Sidekick can be a non-threatening yet incredibly appealing fantasy object for the audience, who identifies with the main hero. A variant of this is when the younger character leads out of status or rank but requires the more experienced sidekick for advice. A typical example of this would beCaptain Smooth and Sergeant Rough.
A related dynamic is when the two characters have a (increasingly) full equal partnership, but there's just enough of an age difference to create protective feelings in the older one. Usually both characters are designed toappeal to different segments of fandom, and the story will end up using many designs that end up appealing to shippers (compareSeme andUke). Think a more intimate version of theBuddy Cop dynamic.
In fact, theOlder Sidekick is very handy for lessening the perceivedSquick of shipping the characters, as the audience seem much more willing to rationalize that the characters are on equal ground.
Tuxedo Mask,Sailor Moon'scollege-age boyfriend. He starts out as theMysterious Protector, but quickly falls into sidekickdom by the end of the first season. Note that he's college-aged in theanime, whereas in the manga he still was attendingHigh School.
Vampire Princess Miyu's minion Larva was eventually revealed to be an incredibly attractive man underneath his mask, and due tofan pressure, we saw it much more frequently.
Renamon and Ruki inDigimon Tamers. Their relationship is more of being actual contemporaries rather than a pet or babysitter dynamic, although this is probably more conductive to Ruki's original cold professionalism.
A Message In The Packet takes it even further: Ruki calls Renamon her big sisterseveral times in her message.
Digimon Tamers actually has a number of these, since the series takes place (mostly) in the real world rather than the digital one. In the latter parts of the series, the kids are backed up by the middle-aged Monster Makers, as well asman in black Yamaki and his Hypnos organization. In fact, their ultimate triumph can be attributed fairly equally to the efforts of both children and adults.
Villain example: Nappa is the older henchman to Prince Vegeta in the Saiyans arc ofDragonball Z. However, Vegeta wasmuch more powerful than him, and quickly killed him when he was unable to fight anymore.
Lumeire and Eclair fromKiddy Grade are also anapparently older/younger girl duo. Many of the partner couples of the GOTT are like this in the series.
Alyssa and Miyu fromMy-HiME. (Well, at least so it seems. SinceMiyu was an android built specifically to protect Alyssa, the latter wastechnically slightly older.)
Evangeline and Chachamaru inMahou Sensei Negima, though only in appearance (hundred-year-old vampire and two-year-old Robot).Negi andAsuna (and hisother partners) along withChamo are definitely played straighter.
Mythical Detective Loki Ragnarok has Yamino Ryuusuke, who looks ten or fifteen years the title character's senior, and is Loki's frequent sidekick, butler, housekeeper, voice of reason...oh, andson.
InChrono Crusade, Rosette tends to introduce Chrono as her assistant. In his usual form he looks to be a young teenager a few years younger than her, so this doesn't seem too odd at first, but in reality he's a demon whois actually closer to 90 years old. In his demon form, he looks closer to a man in his twenties and is much taller than her. And yes, he's very protective of her, and yes, they're the series'Official Couple.
Arf, despite having been created two years before the original series, is "officially" 16 in the original series, having been a familiar long enough to attain an adult form, and tends to consider those in Fate's age group younger than she is.
A straighter example is the Wolkenritter of Hayate, most of whom are not only older than Hayate in appearance, but they're alla lot older than her in age.
Pop fromDai no Daibouken is a few years older than the protagonist Dai.
Despite his 11-year seniority, Kisame fromNaruto acknowledges his junior partner Itachi as the strongest of the duo and does everything he says without question. So that would indeed make him the sidekick.
Though there are many fans who don't want to think of it this way, Kamina ofTengen Toppa Gurren Lagann is really just an Older Sidekick to the true main character, Simon.
They'reBash Brothers who are true equals, both excelling in the areas the other lacks in. Simon later outgrows Kamina, making the latter seem like an older sidekick in retrospect.
Also, the manga gives us Fu, one of Ling's bodyguards. Fu looks 60 or 70, and Ling is only 15. Justified in that Ling is a prince, and Fu's entire family serves the royal line.
Sven Volfied ofBlack Cat is both Train's Older Sidekick and theTeam Dad. His primary role (aside from kicking ass when necessary) is to be the voice of reason, maturity, and emotional stability, asTrain's mental age is about 12 andEve is discovering what emotions are.
Rangiku Matsumoto serves as Toshiro Hitsugaya's lieutenant inBleach, despite her being older and having been a Soul Reaper longer than he has.
Comic Books
TheGolden AgeDCU had the Star-Spangled Kid and Stripesy, his adult sidekick. This was brought back inThe Modern Age of Comic Books inStars and S.T.R.I.P.E and also inJustice League Unlimited, although the potential squick is reduced because her sidekick (the original Stripesy) is her stepfather, tagging along to keep her out of trouble.
Jon Sable's old fencing coach Sonny Pratt appointed himself to this role inJon Sable Freelance.
Another example would be Uncle Marvel, adopted uncle ofCaptain Marvel.
Actually, theentire Marvel Family is older than Billy.
In the 1989-93 NOW Comics comic book adaptation ofThe Green Hornet, Hayashi Kato (sidekick of the second Green Hornet, based on the Bruce Lee role in the 1966 TV adaptation) becomes Older Sidekick to Alan and Paul Reid (the third and fourth Hornets, respectively).
In Books 3 and 4 ofThe Faerie Queene, the only female knight, Britomart, is accompanied by her old nanny, Glauce, disguised as her squire.
InThe Wonderful Wizard of Oz, Dorothy is a child of about 9–12 years old. The ages of her companions tend to vary by adaptation or interpretation. Going by the book, though, Nick Chopper (the Tin Woodsman) was a full-grown man before the Witch's curse caused him to become a steampunk cyborg.
Amped up inTin Man where DG is explicitly stated to be 20 "annuals" old. Glitch and Cain are easily 15–20 years older, the former beingan obviously long-term adviser/old friend to her mother, the latterhaving a son roughly DG's age.
In the third and fourthAlanna books, having won her shield, Alanna goes out to roam the world. Her only travelling companions are her cat Faithful and Coram, technically in her employ, but more or less a father figure.
Artemis Fowl is always accompanied by hisBattle Butler (known, self-descriptively, as Butler). The author justifies the name as the fact that one of Butler's ancestors may have been the person whose name became the title of the profession. This is a case ofDid Not Do the Research, since Butler comes from the Old French word for "wine bottle" by way of the Anglo-French word for cup-bearer.
Modern-day paladin Michael Carpenter plays this role whenever he pairs up with Harry Dresden inThe Dresden Files
This isOlder Than Feudalism: InHomer'sThe Iliad, Patroclus was traditionally an Older Sidekick (and, later, the ultimateDead Sidekick) to Achilles, although modern adaptations such as the movieTroy tend to portray him as younger to be more in keeping with audience expectations.
Former spy Michael Westen ofBurn Notice frequently calls for assistance from his best friend, retired Cold Warrior Sam Axe, who generally enjoys the opportunity for adventure.
Rather odd considering that in the myth, he's Heracles' NEPHEW.
Captain Jack Harkness to the Doctor in theDoctor Who two-parter "The Stolen Earth"/"Journey's End".After being trapped under Cardiff for 2000 years in his spinoff showTorchwood, he's nowtwice as old as the Doctor, who claimed to be 903 around that time.
Though naturally much younger thanThe Doctor, Wilf inThe End Of Time certainly looks the part, being the Doctor's oldest human companion.
In series 5,Rory Williams became this,being an Auton replicant with Rory's memories, standing by the Pandorica for 2,000 years. Though by the end of "The Big Bang", he's nowback in his twenties, with his millennia of memories locked away unless necessary.
In the second module and onward of thePathfinder adventureKingmaker the players need to fill several political positions their new a kingdom. The first module introduces several NPCs, many of whom are of advanced age, that are explicitly suggested for selection. As PCs tend to be young adults by the standards of their race, almost all their cabinet will be older (at least relatively) than them.
K' fromThe King of Fighters has Maxima, who has to keep his younger partner's temper in check at times.
Cloud, the Hero, and Barret, the Lancer, fromFinal Fantasy VII. Barret is fourteen years older than Cloud. Interestingly, neither of them are particularly mature - Cloud spent his formative years in aPeople Jar and has the emotional maturity of a sixteen-year-old as a result, and Barret is emotionally driven to the point of completely neglecting to think his actions through. They seem to cancel each other out.
Metal Gear Solid does the equal-partnership-with-paternity-and-fan-service thing, and (oddly for the series) it's played straight and isn'tdeconstructive. Raiden is theBishonen, and the main character. Snake is aCool Old Guy, the side character, and mentors Raiden. Both arevery sexy.
Both of the sidekickNPCs fromBeyond Good and Evil are significantly older than the 21-year-old protagonist Jade. One of them is her middle-aged "uncle," the other is a soldier in his mid-30s.
Pint-sized sixteenor twenty-one year-old detective Johnny Garland has an older sidekick in the form ofBattle Butler Lenny in the gameShadow Hearts: From the New World. He's also the youngest member of the party. He's not alone, either: Shania has Natan as a bodyguard, and Hilda hasRoger Bacon, although the two of them would disagree on which is the sidekick.
Phoenix's regular partner in theAce Attorney saga is his young assistant Maya Fey, but when he needs extra assistance he calls Maya's older sister Mia fey, who fits in this trope. Also, Mia's assistants were her mentor's Marvin Grossberg and Diego Armando, both older than her. Then Apollo was assisted in his first case by his mentor Kristoph Gavin, and later Phoenix himself, and finally in the SpinoffAce Attorney Investigations, Edgeworth will be helped by the older Detective Gumshoe.
The obscure platformerScaler has theKid Hero Scaler assisted by Leon, ajowly lizard-man with aGiant Flyer. While they don't exactlyget along well at first, eventually they take a shine to each other.Good thing, too, since Leon is Scaler's long-lost father.
Sengoku Basara has two instances of these. First is Date Masamune'sBattle Butler Katakura Kojuro, who has a good ten years on his master and is implied to be somewhat stronger, and second is Sanada Yukimura'sNinja Sarutobi Sasuke. Both examples also play the part of theHypercompetent Sidekick to their rather young and inexperienced leaders, serving as a calmer counterpart to theHot-Blooded teenagers. Incidentally, both masters and sidekicks arerivals to one another.
Donald Duck and Goofy to Sora in theKingdom Hearts series. While they'reFunny Animals who don't really have a set age, they're still pretty much adults.
Baldur's Gate has Jaheira and Khalid, two sensible elves and old friends of your foster father. While you don't have to bring them along (or even meet them at all, if you so choose), they are canonical in the sequel, not to mention that you're given explicit instructions to seek them out.
Fallout 2 has Cassidy. Though his character model lacks the detail to confirm it, he is firmly established to be in at least his late fifties. Marcus would also qualify; his Super Mutant status has made him stop aging, but her served in the Master's army duringFallout 1—eighty years previous.
RL-3 and Charon fromFallout 3 technically qualify (Charon is a nuclear mutant who could be as old as 240, while RL-3 is a model of robot that hasn't been in production for 200 years), but it is more obvious with Star Paladin Cross. Cross mentions to the player that she carried you once, when you were a baby. Jericho will also refer to himself as an old man, especially when the player recruits or fires him.
As in previous examples, Raul and Lily fromFallout: New Vegas. While Raul and Lily have the same age-defying abilities as Charon and Marcus (respectively), they come across as much older because of Raul's constant referring to himself as a feeble old man (and his loyalty quest hinging on it), and Lily's mental instability leading her to mistake theCourier for her grandchild.
Western Animation
Avatar: The Last Airbender: Not only was Aang the youngest member of his trio in the first season (sort of), but the cheerful and experienced General Iroh is the foil to the bitter but complex teenageAnti-Villain Prince Zuko.
InMighty Max, the titular hero is a twelve year old boy so he has hisBody Guard Norman do all the killing. Meanwhile Virgil who is even older, serves part as his boss (telling him what needs to be done) but more as a sidekick, helping him out and giving him details he'll need to know.