Astérix'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.
Quite a few interpretations of Batman. Which actually makes a lot of sense. He was clearly inspired by Zorro (or maybe the Gray Ghost) to become a super hero after his parents' death (and followed through with that childhood plan into adult hood!), keeps tons of cool toys and souvenirs around his mansion, is still taken care of by the same butler he's had since childhood and seems to get along very well with people much younger than him like Batgirl and Robin.
Brewster Rockit, from Brewster Rockit: Space Guy!, 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.
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".
Madman, due mostly to being killed and reanimated with new memories and a different personality. Mentally, he is only about ten years old (at least when his former life isn't poking through).
Nero: 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 often comes across this way, but for a different reason—he's a young boy (or sometimes 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).