City Hunter: Ryo's emotional maturity outside of work is questionable at best.
Claymore has one of the few, female examples. The warriorness Helen is more often childish. She likes and teases other warriornesses, or provokes them even fun. She was so annoyed by Clare that she said Raki would be her "nocturnal toy". Later in the manga you can see them celebrate with other warriornesses. And although all the warriornesses can control whether they let alcohol come to them, Helen is the only one who drinks unrestrainedly because she enjoys it.
Near. He's 17-18 in the post-timeskip (21 in the post-series one shot) and he's always seen playing with toys.
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.
Gohan, Goku's son, have some aspects of this when he's a teenager. How else do you explain the entire Great Saiyaman phase?
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 devour people and is overall a nice guy. Doesn't stop him from throwing tantrum 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.
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, and she is shown to identify with children more than adults post-Heel–Face Turn (despite being in the dragon equivalent of her late teens).
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.
Figure skater Azusa Shiratori is a spoiled 16 year old girl who acts like a 5 year old, she'll steal anything she considers cute even if it's just a food item and she will give it a pet name, if the person who has the thing she wants won't hand it over she will attack them with anything she can find.
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 otakuSgt. Frog is this - followed Pekopon anime as a child, then 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.
Pegasus J. Crawford 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 applied Toon Physics to his cards, making them near unstoppable in the anime and still a serious contender when they were nerfed). 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.