Claymore has one of the few, female examples. The warrior Helen is more often childish. She likes and teases other warriors, or provokes them for fun. She was so annoyed by Clare that she said Raki would be her "nocturnal toy". Later in the manga you can see her celebrate with other warriors and, although all the warriors can control whether they let alcohol come to them, Helen is the only one who drinks without restraint because she enjoys it.
Papi from Daily Life with Monster Girl looks and acts childish, gets easily confused, forgets things immediately, and loves playing video games. She also is shown to identify with children more than adults despite being as old as Miia.
Near. He's 17-18 in the post-timeskip (21 in the post-series one shot) and he's always seen playing with toys.
Katarina in Destruction Flag Otome is a bit of an odd example since she's physically only 8-16 depending on where in the story you are. However, since she remembers the entirety of her seventeen years of living in her past life, she considers herself to actually be 25-34. And yet she still behaves like a hyperactive six year old half the time.
Digimon Tamers: Growmon/Growlmon is just a larger (though notably shorter than Greymon) Guilmon. Growlmon is mentally slightly older, but he carries most of Guilmon's childish personality. Averted with Wargrowlmon, who is pretty snarky and serious.
When there isn't something to punch, Goku embodies this trope, often acting less mature than his granddaughter. Super really plays this aspect up by making him whine about not training and him having petty rivalries with Vegeta like who can cut grass the fastest.
The Ginyu Force would qualify, playing jan-ken-pon to see who gets to fight who, and betting chocolate bars on the outcome of fights.
Fat Buu once he turns good and becomes Good Buu. He no longer devours people and is overall a nice guy. Doesn't stop him from throwing tantrums when he wants to eat and refusing to share his food with others, something that gets him in trouble with Beerus. It was also his innocence that allowed Babidi and his father to take advantage of him.
In Inuyasha you sees the titular hero himself. He looks like a 15-year-old boy, but he's more than 200 years old. However, he does not show a trace of maturity, but sometimes behaves like a child. Even Shippo, who is himself a child, feels sometimes annoyed by this.
Usagi from Junjou Romantica appears to be this. He's actually trying to simulate a child's environment because he wanted to "recreate what normal kids do in their childhood".
Apachai Hopachai in Kenichi: The Mightiest Disciple is known as the "Death God of the Muay Thai Underground" and is a deadly fighter in combat. Outside combat, however, he's very innocent, gentle, and childlike.
King Dedede in Kirby of the Stars, he tends to act childish, constantly whines and demands his way and orders monster to sick on the Cappy's and Kirby For the Evulz.
Ryotsu of Kochikame shows shades of this. He still rides a bicycle, likes things like video games and model kits, enjoys pulling pranks on people and genuinely acts immature most of the time. He can be competent if the situation calls for it, but that isn't often.
Deconstructed with Ilulu from Miss Kobayashi's Dragon Maid. She lost her parents at a young age which, combined with being raised to be an Ax-Crazy Chaos Dragon afterwards, left her emotionally stunted. She is shown to identify with children more than adults post-HeelFace Turn despite being the same age as Tohru. Scenes featuring Ilulu, but not focusing on her, often treat her no differently than Kanna, an actual dragon child who attends elementary school, and Kanna's human friends, also elementary schoolers.
My Roommate is a Cat: Kawase shows a lot of childlike enthusiasm for his age and is generally oblivious to the fact that his friend's cat doesn't like him because he's too noisy and overbearing. Unsurprisingly, when children end up being in said friend's house at the same time as him, Kawase joins their games without a second thought and gets along with them simmingly.
Misato from Neon Genesis Evangelion. Downplayed, and played for tragedy instead of humour. Her flighty, immature behavior when she's not on duty is a cover for serious emotional issues, and hinders her ability to connect to her wards and pilots, Shinji and Asuka.
Luffy from One Piece is either one the top codifiers of this trope or a Ditzy Genius with a playful way of approaching things, either way the guy is a infant trapped in a young adults body always thinking about the most childish things and has no mature aspects outside of fighting and being a beacon of Heroic Determination for the crew.
Hinako Ninomiya is somewhere in her mid-to-late twenties; however, thanks to Happosai, her body ages incredibly slowly, so she currently looks like a ten year-old girl. She can only grow up to reflect her true age by absorbing the Battle Aura of highly-combative people around her, and even then, only temporarily. How she acts depends entirely on how she looks — she could be watching Doraemon, reading Shojo or gushing over the giant panda in the living room as a child one second, only to suck out Ranma's ki and try to hook up with Akane's father as a grown, mature woman the next.
Happosai's general outlook on life can be summarized roughly as "five year old child with the hormones of a stereotypical boy just hitting puberty".
Atsushi in Recorder and Randsell is an unusual variant - he really is a child, but looks like a grown man.
Galactic otaku Sergeant Keroro of Sgt. Frog is this. He followed Pekopon anime as a child, then was subjected to Training from Hell until he was assigned to Pekopon to lead the invasion... making it far easier to follow his old hobbies as a result. Just picking up where he left off in life.
Naki from Tokyo Ghoul. Emotionally immature, easily confused, and surprisingly childlike when he isn't being psychopathic. He frequently misunderstands conversations, is prone to screaming/crying meltdowns, and at one point proudly explains that he is writing a letter to his beloved Cool Big Bro in Heaven.....and it will be delivered as long as he addresses it to "Heaven". While an antagonist, he is portrayed as one of the most sympathetic characters due to his childlike nature.
The title character of Tonari no Seki-kun. A high-schooler with a lot of imagination, who spends all of his school time playing with various toys, or doing other things that have nothing to do with schoolwork.
Yotsuba&! has Yanda. The chapter introducing him shows him having petty arguments with the 5 year old Yotsuba, aiming childish insults at her, and taking a big bite out of her ice cream. This and later chapters portray him as a major moocher when it comes to food and vacation time, and the chapter where he tags along on a trip to a ranch shows him to be just as easily amused by the animals there as Yotsuba. This is all despite the fact that he's a grown man and the coworker of Yotsuba's dad.
Pegasus J. Crawford/Maximillian Pegasus from Yu-Gi-Oh! despite being Younger Than He Looks is a large fan of cartoons, talking about them fondly in his youth to Kaiba and vividly recalling all the episodes. This led to him using a Toon deck, though being the creator of the game, he gave his Toon monsters effects based on Toon Physics, making them near unstoppable in the anime and while they were nerfed for the actual card game (and later episodes), they are still a serious contender. That being said, Pegasus has also his serious and dark side, which is reflected by some of his other cards, especially Relinqueshed and Thousand-Eyes Restrict.
Tae from Zombie Land Saga is an interesting variant. As time goes on, she goes from being mindless to demonstrating curiosity and interest in the world around her, albeit in a very basic manner. In essence, being a woman child is a step up for her.