In most cases, the mentor has many years of experience in the topic their student is taking lessons for. The reason for this is often because they are older and have seen more than their young disciple.
That is not necessarily the case here. For this instance, the mentor is actually a few, or even many, years younger than their disciple. The precise reasons for this may vary, but it always comes down to the fact that the younger person grasps the given subject matter better than the older student on a theoretical and/or practical level, making them a valuable source of knowledge. Think about how in Real Life, younger individuals may often find themselves teaching the older people in their life how to use newer pieces of technology and you get the idea.
Something to note is that if the mentor looks younger than their pupil, but is actually older than them, it is Not This Trope. That being said, this trope can come into play from the opposite spectrum, especially if the student looks younger than the mentor but is Really 700 Years Old.
- Lyrical Nanoha:
- Due to her status as a Child Prodigy, Nanoha was a Veteran Instructor teaching advanced areal tactics to classes of full grown adults before she was even a teenager.
- Fate takes on Teana as an apprentice at the end of StrikerS. Chronologically, she is three years younger than Teana due to her status as a clone, though her legal and mental age are older.
- Being Born as an Adult, Nove is actually the same age as most of the members of Team Nakajima, and two years younger than Einhart. Likewise, Corona and Rio end up studying under Otto and Deed (who are six years their junior) in the lead up to the Inter-Middle Championship. Though like Fate, they are legally older.
- While she is primarily Einhart's student, Fuka does receive some training from Vivio, Rio, and Corona (who are all a year younger than her).
- Mahou Sensei Negima! starts off with this as it's premise, with the ten year old Negi teaching middle school English to a class of Japanese schoolgirls. He also mentors some of them in magic once the Genre Shift kicks in.
- Overlord (2012): Ainz is technically in his twenties, but his body is now that of an immortal lich. He ends up teaching magic to The Archmage Fluder, who's over two hundred years old (or rather, since Ainz was just dropped into his MMO character and has no idea why he has magic or how it works, he gives Fluder some books he found in the guild treasury that talk about magic).
- In the epilogue of the Batman fanfiction, "Anarchy for All," thirty-something Selina Kyle asks twelve year-old Damian to teach her how to sword fight as a way to bond with him now that she's engaged to his father. He gets more into it than expected, channeling a Drill Sergeant Nasty and ordering her to call him "Master." Either Damian is a great teacher or Selina is a prodigy, because two volumes later, she can keep up in a duel with Damian's Evil Aunt Nyssa, albeit still needing dirty tricks to actually beat her.
- Wizards of the Lost Kingdom 2 concerns the Child Mage Tyor learning magic from the Inept Mage Caedmon, then defeating three evil wizards to restore peace to the kingdoms. At the end, after the evil wizards are dispatched, Tyor receives a new mission from the great wizard Vanir: to re-train Caedmon, his former mentor.
- In Star Wars Count Dooku, who was in his eighties, was this to Palpatine, who was in his sixties at the time. However, Palpatine sought to avert this after Dooku has outlived his usefulness and sought a far younger apprentice in Anakin, and he himself would soon look far older than Dooku after his fight with Mace Windu.
- Jacob's Ladder: Jacob's deceased toddler son Gabe guides him to Heaven, now infinitely wiser from his time in the Afterlife.
- Animorphs: After the Yeerks are defeated, Jake ends up teaching American soldiers how to morph. He hasn't even graduated high school, but everyone (including people twice his age) calls him Professor.
- Circle of Magic: Tris, a teenager, ends up teaching an adult, in magic, as she's the one who discovered him.
- In Journey to the West, Tang Sanzang is a mortal monk in his twenties or thirties at oldest, while his three disciples are all immortal demigods who are centuries old, with Sun Wukong being over 500 years old due to his imprisonment under a mountain as punishment.
- The Wheel of Time: Nynaeve joins the Aes Sedai Magical Society at age 25 to learn Healing powers, but soon realizes that one of her Achievements in Ignorance as an untrained Channeler was to improvise a Healing variant that's far superior to the Aes Sedai method. A few Aes Sedai — some of them a century or two old — swallow their pride and come to her for guidance.
- In Star Wars Legends, in the early days of the New Jedi Order Luke Skywalker had some students who were significantly older than he was, sometimes by decades as in the case of Streen, but had slipped under the Old Jedi Order's radar and were just as inexperienced as the younger students as a result.
- The Red Tent: Meryt states many times that, despite being significantly older than Dinah and having more practical experience, she considers herself Dinah's apprentice in the arts of midwifery, owing to the latter's more advanced skills.
- "L'Alchimista" by N. K. Jemisin: The elderly Mysterious Stranger who visits Supreme Chef Franca's kitchen eventually asks to become her apprentice, even though he's not personally interested in the culinary trade. He's so long-lived that the rules of Alchemy have changed underneath him, and he needs her fresh, new perspective to keep the Art alive.
- In The Bible, Jesus ends up teaching Jewish law while still a child.
- In Dragon Ball Xenoverse and its sequel, Gotenks can serve as a mentor for the protagonists. While no age is specified for him/her, it is safe to believe they are older than Goten and Trunks. Pan (specifically her 10-year-old GT incarnation) also counts, and Xenoverse 2 gives us Kid Gohan, at the age he was during the Saiyan to Planet Namek sagas, as a possible mentor.
- In Overwatch, Zenyatta is the younger mentor to Genji. Zenyatta is only 20 years old, but, as an Omnic, he was essentially Born as an Adult.
- Downplayed in How to be a Werewolf: Elias is a 22 years old born as and to a werewolf among werewolves, serving as an aide and personal agent to his mother the alpha of a fairly important pack. Malaya is a 25 year old quasi shut-in who was bitten by a mysterious beast as a five year old girl and whose parents managed to teach her self-containment despite having to work things out as they went along. He caught her scent while she was behind the counter at her father's coffee shop, determined she was not involved in the murders he was investigating, figured out how clueless she was, and offered to help.
- Avatar: The Last Airbender: Katara, Toph, and Zuko all serve as Aang's mentors to learn all four elements. And while Aang is physically and mentally twelve, he was frozen in ice for 100 years, making him older than all of them. Even if you don't take his actual age into account, Aang is still slightly older than Toph.
- In Avatar: The Legend of Korra, Jinora, Avatar Aang's grandaughter, is Korra's Spirit Guide. While it isn't a huge age difference, Korra is in her late teens, and Jinora is pre-pubescent.
- My Little Pony: Friendship Is Magic: Following the episode titled "Marks For Effort", Applebloom, Scootaloo, and Sweetie Belle become tutors for Twilight's School of Friendship. Keep in mind that they are still fillies and the school has students of various ages. This means that the Cutie Mark Crusader are not only the youngest of the school staff, but they are also younger than some of the students attending the school.
- Truth in Television as it is highly likely for Technologically Blind Elders interested in learning to turn to their younger peers to teach them how modern devices work.
- For older adults who are returning to education to finish a degree or gain new skills, this can be the case, with them being their 50s or even 70s while the professors are in their 30s.