L sits like a kid, eats a lot of sugar, playing with his food, and in the anime, after hearing something important, ran and dove head first on the couch like a ten year old boy would.
They give the reason for his sitting position being that the brain gets the most oxygen from that posture. His liking of sugary foods has been explained in special material as being necessary for his brain, which burns calories like mad.
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. Growmon is mentally slightly older, but he carries most of Guilmon's childish personality.
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 innocents that allowed Babidi and his father to take advantage of him.
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.
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.
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.
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 the pilots in her care, especially Shinji.
Yato from Noragami. Biologically in his early twenties, chronologically over 400 years old. It's not his primary characteristic, but Yato is often very playful and sneaky, often doing silly and childish things such as throwing a tantrum on the floor when Yukine says he doesn't want to go with him to Capypa Land. However, do not piss him off, as he can go from Adorkable to terrifying in a heartbeat.
Figure skater Azusa Shiratori from Ranma ½ 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.
Atsushi in Recorder and Randsell is an unusual variant - he really is a child, but looks like a grown man.
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 sad, Pegasus has also his serious and dark side, which is reflected by some of his other cards, especially Relinqueshed and Thousand-Eyes Restrict.