Asterix's best friend, Obelix, has a very innocent attitude to life despite his superhuman strength. He sulks when there's no wild boar (in Asterix in Spain he copies little Pepe and holds his breath until they get some); he can't think in the long term (eats the whole boatload of food on the first day at sea); and he only ever drinks goat milk.
Batman is sometimes interpreted as a competent, downplayed variation, which is understandable. Zorro inspired in him the idea of becoming a superhero, and he never gave up on that plan. Alfred Pennyworth continues parenting Batman even after the latter reaches adulthood. He keeps tons of toys, and gets along very well with people who are much younger than himself, such as Batgirl and Robin. Furthermore, nearly all of his regular antagonists have immature dispositions.
Brewster Rockit: Space Guy!: Brewster Rockit is shown to be a manchild, even having book tapes on coloring books, and being exceedingly stupid. It is implied that he originally had average human intelligence, but his intelligence and maturity decreased substantially due to the government overdosing him on memory wipes.
Deadpool ventures in and out of this, Depending on the Writer. In some cases, he is a genuine man child whose combination of serious psychiatric issues combined with a brain that is constantly regenerating itself leave him someone with attention deficit disorder who is obsessed with collecting toys from Happy Meals and will drink Drano just for the hell of it. In other cases, it's all a carefully planned act to get people to dismiss him as a lunatic and not realize what a lethally efficient and dangerous mercenary he is...until the bullet goes into their brain, the katana sword goes into their heart, or the well thought out plan you had no idea he was executing is played out right under your nose. Characters who are either smart or know him are aware that he is someone you do not want to underestimate, no matter who is doing the writing.
In Empire State, Jimmy admits that he really doesn't feel grown-up, even though he's 25. But when his friend Sara calls him out for not having a checking account, and still receiving an allowance from his mother, he insists that "it's an Asian thing".
Nero, who still has a very childish way of looking at the world. For instance: he is the only adult who still writes a letter to Sinterklaas.
Abraham Tuizentfloot in the same series is also a good example. An adult who dresses like a pirate and has very infantile fixations. Of course, it doesn't help that he's also completely mad.
Similarly, Rage of Marvel's New Warriors was an immature young teen when he got his powers — which mutated him into the form of a very large and muscular Scary Black Man.
Captain Marvel (of DC fame, formerly Fawcett) often comes across this way, but for a different reason—he's a young boy (occasionally young teenager) named Billy who can transform into an adult superhero. Early on the two forms had different personalities, but most modern interpretations make them the same person, acting like a Cheerful Child in both forms (though Marvel gets a bit of maturity from having the Wisdom of Solomon as one of his powers).
Superman antagonist Mister Mxyzptlk is an enormously powerful Reality Warper from the Fifth Dimension. But he's obsessed with playing "games" with Superman more than anything else, throwing tantrums when he loses or when Superman is too busy to deal with him. When he briefly takes over Clark Kent's life, his kitchen is filled with sweets, and when he returns to full power, the reality he manifests is full of toys and candy, and he throws a nightmarish tantrum when Superman and Lois get closer and closer to finishing the game.
Spider-Man is this depending who's writing him and how well thought out the story is, most basic tropes of the hero apart from the other heroes is despite being as old or younger as a adult as them in most versions, he's mostly the Overgrown Teenager of the Major Marvel Hero names, and is quite possibly the most famous one of them all after Batman. Also, like Deadpool, Spidey acts like a young and nerdy free spirit and babbling immature goofy ball as a way to handle his fears in being a crime-fighter and to cope with the various tragedies of his past. The difference between him and Wade is that Peter usually operates in behaving intentionally to deceive threats, though he still shows he can be the awkward nerdy teenager at heart. However, this endears him to the younger generation of heroes, who all adore and look up to him for starting the game at their age and being a success. Mind you, he is perfectly capable of being serious and it's a rule of thumb that if Spidey is not making wisecracks or even talking, he is not playing around, to where some of his colleagues are scared as well.
Van Zwollem, a mad middle aged man who enjoys playing outside like an infant.
In the early albums Jerom also had an infantile personality, but as he became more civilized he lost this character trait.
Minor Marvel character Wundarr had this as his defining characteristic trait when he debuted in the pages of Man-Thing. A Captain Ersatz of Superman, when his parents misguidedly rocketed him away from their planet, which turned out to not be doomed after all, his pod landed in the Man-Thing's swamp where nobody encountered his pod for 23 years (save one suspicious old couple who refused to mess with it). As a result, whilst Wundarr's body aged to maturity, his mind never developed beyond that of a baby. When freed by the Man-Thing, Wundarr thus acted like an infant in the body of a young adult. After being adopted by the Thing and Namorita, Wundarr eventually managed to mature beyond this limited state.