The title character of Billy Budd is a textbook example.
Zhou Botong of the Condor Heroes martial arts novels is the Trope Codifier for Chinese literature. Despite essentially being the co-founder of one of the story's pivotal martial arts sects, he'll sneak off for fun at any given opportunity, making friends with the much-younger protagonists while imparting his unique brand of wisdom to them. Sex as Rite-of-Passage is averted early on, as the fact that he sired a child with an imperial concubine, while his fellow disciple (the founder of said sect) imparted a powerful technique to the Emperor, not only forms part of his backstory but arises later as an important plot point. On top of that, he's incredibly long-lived and one of the novel's most powerful characters.
Charlie from Flowers for Algernon is a mentally disabled man in his thirties who remains very childish, naive and idealistic. He had also never had wet dreams or been attracted to women at the beginning of the book, although the realism of this aspect of his disorder is somewhat doubtful.
When he starts remembering things later on, he can remember being aroused by girls when he was little, which ended with his mother's traumatizing responses.
In Death: Poor Alice Lingstrom from Ceremony In Death turns out to be a child in a woman's body. She was just exploring Satanism and got involved in a Satanic cult. The cult drugged her and then gang-raped her. She was made into the cult leaders' slave for a time, but she left when she witnessed the two leaders sacrifice and murder a young boy. She actually thinks a spell had been cast on her, and that one of the leaders is a shapeshifter. Considering that she is suffering from trauma and paranoia, it is safe to say that her status of Womanchild is being Played for Drama.
Bertie Wooster from Jeeves and Wooster. His valet is implied to be acting as his Parental Substitute, and he's very childish in general—loves playing in the shower "like a two-year-old", doesn't know how to deal with women, and never got over his fear of his Grande Dame aunt.
The books The Heroes of Olympus have Percy Jackson. He is a teenager and not an adult, but still behaves rather immature. Nico di Angelo notes that the older he gets, the more he realizes how childish Percy really is.
Kino of Kino's Journey turns out to come from a country where all adults are examples of this — they're given surgery to become adults, rather than allowed to gain genuine maturity and wisdom. Their apparent adulthood turns out to be nothing but the thinnest of veneers, and any real stress or deviation from routine reveals them to be little more than children in adult bodies, going through the motions of maturity. The scene where they childishly fumble for the "mature" way of dealing with one of them murdering the original Kino in what amounts to an overgrown temper tantrum is horrifying.
This is exactly how General Thiébault describes General Junot in his Mémoires ("At heart, he was not a man but a child, who could age but never grow up"); he gives spectacular examples of his extravagances and general immaturity, which he mentions as the main cause of his disgrace. Having read the secret diary of Junot's own wife, the Duchess of Abrantès (in which she recounts how he flew into a jealous rage and attempted to rape and murder her upon discovering her affair with Prince Metternich), modern authors tend to put him more on the psychopathic side instead.
Lennie Small from Of Mice and Men is a deconstruction; he's childlike due to being developmentally challenged, but since he combines a small child's brain with the body of a huge, powerful man, well... things tend to turn out badly.
Dorian Gray, from The Picture of Dorian Gray seems like one of these when first introduced. His innocence is obvious, and he is naive and gullible enough to be influenced by his friends to adopt their lifestyle right away, without any opinions of his own. There's also the fact that throughout the novel he's referred to as a "boy," "lad," or "youth," even though he is at least eighteen.
Upon realizing he won't stay pretty forever, his response is basically to sob into a pillow.
In the Rainbow Magic series, Jack Frost sleeps with a teddy bear, is scared of the dark, and on the whole is very petty and childish.
Sisterhood Series by Fern Michaels: The Vigilantes seem to act like womenchildren a number of times. At least Myra Rutledge and Countess Anne "Annie" de Silva have the excuse of being rich 60-something-year-old ladies who may have never developed maturity...or they lost it as they got older! Cosmo Cricket, introduced in Final Justice, could be considered this. However, he has wisdom and is quite responsible in his job as a lawyer!
Tom Cullen in The Stand (due to being mentally disabled). He's middle-aged, but enjoys playing with toy cars.
Speaking of Cullens, Edward of Twilight is over 100 years old, a virgin, and decides to start a relationship with a human whose blood smells sweet to him and his presence puts her in constant danger. He gets better, although she gets MUCH worse.
Bella herself counts towards being a woman-child with her personality usually ranging from doormat to manipulative sociopath with her manipulation of Jacob and Edward.
At least she's an immature teenager (despite Informed Ability claims to be mature), albeit even more selfish and perspective-challenged than most. What's Edward's excuse?
When one becomes a vampire in the series, they more or less stop developing, physically and possibly mentally. Since Edward was a young man when he got the Spanish Flu and converted, he hasn't aged since then.
Tigger comes off as this compared to the other animals in Winnie-the-Pooh. He's even sillier and more irresponsible than the other characters, he lives with Kanga, and he's best friends with her infant son, Roo.
He appears older, but when you consider that he "came to the woods" (i.e. was purchased) more recently than the other characters, he's technically younger even than Roo.
Lieutenant Wes Janson of the X Wing Series is a classic example, between his pranks, general irreverence towards everything in life, and his boundless sense of humor. He is capable of being serious, when actively shooting at things, but it's not his natural state. However, he explains that this is a deliberate part of his philosophy of living life to its fullest, given the mortality rate of his chosen profession.
Janson: I want you to remember something very important: you can't look dignified when you're having fun.
Myn Donos: I... I'm asking for career advice, from a nine-year-old.
In Warbreaker, the God-Emperor Susebron turns out to be so sheltered by his priesthood that he thinks he can conceive a child by sitting in the same room as a woman. Justified in that the high-ranking members of the priesthood were terrified of what he might do with his phenomenal Awakening powers if he knew enough about the world to take an active interest. Once he gets to spend time around people who treat him as a person instead of an object of worship, he proves to be a very precocious student.
Sharon from World War Z is one of the more tragic examples. She's one of the "ferals", young people who grew up with little (if any) human contact after losing (or being abandoned by) their parents during the Zombie War, and has mentally never progressed beyond the level of a young child. However, she has retained some language skills and is able to describe the events which led to her living wild, albeit in a very simplistic way.