For the most part, as someone grows older they become more mature and grounded in reality. They pass by the immaturity of their younger days and become an adult.
But there is a phenomenon where in a specific situation a person may turn from their confident adult personality and revert into a nervous teenage (or younger) incarnation. It may be because of interacting with an old friend and they reminisce about all their old activities they did together. Or maybe they have found themselves re-enacting a situation from their past they thought they had grown past, such as breaking up with a long-time partner and finding themselves back in the dating pool. Then there are the supernatural and "bump on the head" reasons.
A common gag with this involves a Parental Substitute where a "more mature" friend may have to be the voice of reason and they bicker with things like "You don't understand me!"
- Expelled from Paradise, Angela Balzack (and 98% of humanity) has been digitized, quickly followed by the death of her original physical body, when she was about 2 months old. When she has to go back to the real world, she isn't patient enough to wait the whole 36 hours required for a new body to be cloned and aged to her chronological and mental age. So she has the computer cut the time short, resulting in her body having the physical age of 16. Upon landing in the real world, she shows remarkably decreased maturity, possibly because the virtual world she spent her whole life in doesn't simulate hormones, and since she's a teenager now... yeah. Add in that the virtual world probably doesn't simulate periods and she's stuck IRL forever now, she's gonna have fun in a couple weeks.
- Maison Ikkoku when Kyoko returns home to stay with her parents. Kyoko, who has always been a mature and feminine character, gets into arguments with her father, and begins sitting and speaking in a boyish fashion. Apparently Kyoko was quite the tomboy in her teenage years.
- The main premise of Nanaka 6/17 involves the title character hitting her head and then waking up with the mental age of a six-year old.
- Happen in one episode of Sket Dance. In the episode, Bossun got his body reverted back to a five year old's after drinking Chuuma-sensei's potion. At the end of the episode, Chuuma-sensei made another potion that returned his body to normal... but now it's his mind that reverted to a five year old's.
- In Hamatora, Honey has the power to see ten minutes into the future after biting down on a lollipop. For much of the series she can use this power without repercussion but later her ability develops a side effect that causes her to mentally regress to childhood for a time after her power runs its course.
- In Noir Mireille is walking down the street when she hears someone call out her name. She reflexively tenses up and turns around with her typical Ice Queen expression, being a professional assassin by trade and clearly expecting the worst, but as soon as she sees it's her Uncle Claude her face lights up and her body language becomes much more relaxed and open as she talks to him. Since he took care of her ever since the death of her parents she's likely reverting to her childhood persona.
- Briefly poked at in The American President, where widower President Shepherd found himself attracted to a lobbyist and sought to find some way to pursue a relationship with her without causing too much attention. His attempts at finding some information out of his Chief of Staff (and best friend) ended up making him sound like a middle-school kid wondering if a girl liked him.
- In One Flew Over the Cuckoo's Nest, one of the patients is chronically shy and soft-spoken, an emasculated young man with an intense fear of his mother and an unrelenting stutter. With the help of McMurphy, he manages to have sex for the first time, losing his virginity and gaining the confidence of a man. However, they were supposed to escape from the mental hospital and ended up falling asleep. When the emasculating and manipulative head of staff, Nurse Ratched, returns, she calls him out and attempts to shame him. He says in perfect and unhesitant English that he's proud of himself and isn't having any of it. Then she says she's going to tell his mother...
- In the Aunt Dimity books, Lori's husband Bill says that when he was twelve years old, his mother died, and he reacted by starting to suck his thumb. Dimity tells Lori that's a common reaction to stress in a child that age.
- Happens to multiple characters in the Discworld book Hogfather. Justified as the characters have essentially crossed over to Another Dimension that the tooth fairy runs, (really) and this is making them regress somewhat to their childhood. It also has some deadly side effects, like making their childhood fears come to life and come after them...
- In Dragon Bones Oreg, who looks like seventeen, but is Really 700 Years Old often acts like a frightened child. Justified, as he has spent his entire not-life as slave, which makes his seemingly childlike "Please don't be angry with me" Fridge Horror.
- P G Wodehouse's older characters often carry on like juvenile delinquents. It might be possible to regard this as an aversion of the trope, however, since such characters are usually perfectly aware of how they are regarded, and are simply too self-confident to care.
- In Artemis Fowl: The Time Paradox, during a Time Travel trip, Holly is turned into her younger self. Her emotions are affected too, such as getting jitters on a mission that her older, more experienced self wouldn't have.
- Addressed in full in Home Improvement where a college friend of Tim's was in town, who was not liked at all by Jill. Tim realized that all their time together is spend reminiscing about their "glory days" and trying to re-enact them. Since Tim had a family now he couldn't go off to a bar like his friend wanted to do and the overall message was that true friends grow up with you, which his college buddy did not.
- How I Met Your Mother:
- Made into a major Running Gag when Robin's first boyfriend came to visit and despite still being in the same band with no music label backing them and working at a water park to support himself, Robin reverted to her starstruck teenage personality. This lead to a discussion among the group about others who do the same thing, such as Ted having a playful push-fight with his high school buddy Punchy and Lily speaking in heavy street-slang when she meets up with her friend Michelle.
- After his father dies, Marshall goes back to Minnesota and lives in his old room for a while. He quickly starts acting like a teenager: playing '80s and '90s video games, calling to his mother to bring him chocolate milk and snacks, and so on. Ted goes to Minnesota to snap him out of it (and avoid making a big decision, but that's another story), and ends up acting exactly the same way. It takes a visit from Lily to snap both of them out of it.
- An episode of Titus had Titus talking with Erin's teenage niece Amy and trying to console her over a break-up (and subsequent suicide attempt). Knowing there is only so much adult wisdom he could give her, Titus in the "Neutral Space" dug deep to find his teenage self and the long haired, flannel wearing, 17 year old stoner version of him narrated from then on.
- The Twilight Zone (1959) episode "A Piano in the House": A player piano with Mind Control abilities causes a woman's personality to revert to that of a little girl.
- In the Angel episode "Spin the Bottle," an attempt to fix Cordelia's amnesia resulted in the entire group sans Lorne thinking they were 17. Thus Cordelia reverted to her Alpha Bitch personality on early seasons of Buffy the Vampire Slayer, Fred started craving pot. Angel was 17 years old some 200 years ago, and it was a serious culture shock.
- An extreme example in the United States of Tara, one of Tara's "alter egos" is called Gimme ("give me")—an animalistic, greedy, flighty child-like version of Tara that comes out any time she feels emotionally cornered/blackmailed. It does such immature things as scream like a baby, or pee on her sleeping parents. There's also "T", the teenager alter ego, who does things like make out with her (gay) son's date.
- By and large Sheldon from The Big Bang Theory is already a Manchild, who refuses to drive or shop for his own food (which is really best for all concerned, he is easily distracted and insufferable when it comes to getting his way). But in particular moments he very much becomes a kid including when he is playing with trains or regressing when Leonard and Penny fight because it reminds him of his parents fighting all the time.
- While Mac from It's Always Sunny in Philadelphia displays a lot of Manchild traits, he becomes very pathetic whenever his parents are involved and starts talking about things like having a catch with his dad and wanting his parents to get back together so that he could be a happy boy.
- Lymle from Star Ocean: The Last Hope is actually 15 years old by her race's standards, but is both physically repressed and shares the mentality of a 6 year old as a result of her symbology magic briefly opening up and sending her into a Hell-like realm of fire.
- El Goonish Shive: Tedd's childlike reaction to the noise of the magic analysis wand turns out to be a case of this, since his parents used it frequently on him as a baby to see why he didn't have any magic potential.
- In Elena of Avalor, Elena's cousin, Chancellor Esteban, can act rather immature for a fifty to sixty year-old politician, such as being willing to mess up Elena's negotiations with a visiting kingdom to be petty or dismissing Naomi as an uneducated and untalented commoner. In the pilot movie (released after the show premiered), we're shown that he wasn't always like this. Given that the pilot ends with his grandparents and cousins essentially returning from a magical coma without having aged for the past four decades (not to mention a possible Inferiority Superiority Complex), it makes sense.
- "Snow Place Like Home" even addresses this, where Esteban's lack of maturity gets called out on. He reveals that this stems from guilt over whether or not he could have prevented his parents from dying in an accident in his youth and he fears losing the rest of his family as well.
- A variation occurs in the Justice League episode "Kids Play." Batman, Superman, Green Lantern and Wonder Woman are physically reverted to pre-teenage bodies and accompanying personalities, but otherwise retain their memories of their adult life. Thus Wonder Woman becomes a Team Mom, a role she otherwise doesn't fit, and was flirty towards Batman, Green Lantern struggled to contain his childhood imagination and Superman was even more naive and farmboy than before. Amusingly, Batman is the only one who retains the same basic personality and when challenged, he points out "I haven't been a kid since I was eight."
- There was a Popeye cartoon where Popeye started acting like a baby himself after Swee' Pea was kidnapped.
- Star vs. the Forces of Evil: In "Tough Love", Eclipsa attempts to stop Meteora's rampage by approaching her with her old teddy bear Bobo and speaking to her like a small child. It actually works, but when Eclipse refuses to take the throne of Mewni alongside her, Meteora stomps off like a child and Eclipsa shouts after her like a scolding parent.
Eclipsa: Young lady, you turn right back around!