Never mind the fact that he's thirty years old
Sometimes people just fail to develop into social or intellectual maturity. Maybe it's due to a very loving but confining mother or father, who don't suggest or won't let them get a job and move out, enabling their laziness or dependence instead. Maybe they were never taught any responsibility growing up, and now that they're adults, it's too late and they're set in their ways. Maybe they're a Spoiled Brat who's learned to manipulate their parents into doing what they want, and the parents came to the conclusion that it's just easier to indulge them than to fight about it. Perhaps they just didn't want to leave the nest. Maybe they've been intentionally secluded from learning about the world. Or they grew up as a neglected orphan without a proper education or any guidance towards adult life. Possibly their parents or legal guardians just failed to teach them how to do things like cook for themselves, keep their home clean, balance the checkbook, make and stick to a budget, pay bills and taxes, do minor home repairs, or change a tire. Maybe they were literally Raised by Wolves. It could be it's the result of brain damage, an intellectual disability or developmental disorder, or something sinister. Perhaps they just never had a life-changing moment involving a shotgun and a beloved pet. Maybe it's just a form of Mars and Venus Gender Contrast, or they live in a society with prescribed gender roles, in which one sex is expected to be the responsible one, and the other is not expected to be (and is not always taught to be). Who can say?
Although the causes might not be clear, the effects will be. The Manchild, a term invented by William Faulkner, is usually an adult who possesses a very childlike or (stereotypically) childish demeanor. He's emotionally both simple and fragile; he prefers (though does not always need) to have a parent figure to look after him. He usually isn't terribly worldly and is typically pretty gullible. The Manchild's interests tend to be what most people regard as immature or childish, sometimes even in comparison with actual children. Psychologically speaking, this is very much linked to the defense mechanism of regression
(since it involves mentally reverting back to earlier stages).
The character is almost Always Male. This is (presumably) to contrast the differences between him and "normal men" with the normal responsibilities and wisdom of adulthood. The female version usually overlaps with The Ingenue, the Manic Pixie Dream Girl or the Genki Girl. Manchild has many more negative connotations than The Ingenue; the manchild's immaturity and lack of outward adult behavior is emphasized as being a bad thing versus being an emphasized good thing like The Ingenue's purity and idealism. This is also because of the Double Standard most societies have, which exist for various reasons, where they expect more from men than they do from women, due to the latter being perceived as inherently weaker and less responsible than the former. In other words, it has historically been more "expected" for women to be developmentally stunted compared to men, and therefore less outrageous. More contemporary works are increasingly likely to treat the genders equally and play female examples as unsympathetic (or, at the very least, deeply flawed), with a characterization that is fairly similar - irresponsible, unreliable, unable or unwilling to meet basic adult responsibilities on their own, and dependent on others. When portrayed unsympathetically, female examples are typically more dysfunctional than male examples, as their parasitism is typically aimed at partners (usually viewing them as meal tickets and lifestyle subsidizers, and often cheating on them when they grow bored with them or find a better provider), and they also tend to have more severe mental health issues and are more likely to have serious issues with substances.
On the Brain Chain, the Man Child occupies a space between The Cloud Cuckoolander and The Ditz, but without necessarily becoming The Fool. He usually does not have The Fool's luck, but he doesn't necessarily play the role of the Butt-Monkey either. Although the Manchild is commonly portrayed as being mentally challenged, he does not necessarily have to be. In many cases, the character may be very intelligent, and even leave the idealism aside and be very shrewd in business or career. Alternatively, his childlike qualities/way of thinking, when intelligently applied, can be a basis for his success as a businessman, in which case he's also The Wonka.
In comedic works, he usually plays the role of The Ditz. In dramatic works, he could be the Jerk with a Heart of Gold due to his simplicity or immaturity, or an Audience Surrogate. Sometimes the Man Child embarks on a late-in-the-day Coming of Age Story, which ushers him into true adulthood. Note that usually Sex as Rite-of-Passage works only some of the time. In many instances, a Man Child is not necessarily a virgin, but only sees sex as a tool of pleasure and does not recognize its emotional significance. If he ever gets married, it's likely that his wife will end up being a mother-figure not only to their children but to him as well, doing all the "emotional labor" needed to keep the household afloat while he just coasts along and drags her into all sorts of wacky hijinks. If he doesn't, there's a good chance that he's a Disappeared Dad - he very likely knocked up an ex or casual sex partner, but plays no role in the child's life whatsoever, either because he can't or because he won't (quite often both). The Manchild is occasionally prone to delivering or enjoying a "Life Is Awesome" Speech.
One of the Kids is related, in where their childishness is caused by spending a lot of time around children. When events will force him to be independent, he will be Not Used to Freedom.
Does not relate to Never Grew Up, because they physically did grow up - but never outgrew being attached to immature or childish things or behavior. Sister Trope (perhaps) to Adults Dressed as Children, although that trope is almost always played for laughs or borders on the grotesque. Compare Keet, One of the Kids, Kiddie Kid. For a villainous version, see Psychopathic Manchild. Compare Basement Dweller. May form a Nerd Hoard out of items relating to their childish obsessions. Not related to Manchild (2002), the British TV series, either. Compare and contrast The Three Faces of Adam. Contrast with Wise Beyond Their Years, where a child acts like an adult. Also contrast Feigning Adulting, where someone doesn't feel mature, but presents themselves that way to set a good example for others. Can overlap with Only One Finds It Fun for when the manchild is entertained by children's entertainment that fails to entertain the actual children. May engage in the Jinx Game as an adult and expect others to abide by the rules. Also likely to be a Holiday Aficionado, such as a Halloween Enthusiast. They're also likely to partake in Re-creating Musical Shout-Outs.
Adult women may reveal such tendencies with the use of the Feminine Leg Swish.