The Earl of Lemongrab is an interesting version of this. Physically he's eighteen at the youngest, but because he was born in a laboratory, he's only about one year old chronologically. He acts like a bratty little kid trapped inside the body of an adult, mainly because he IS a very young brat who happens to be an adult, too.
He and his clone/boyfriend have kids together... and they're STILL immature manchildren.
Jake also very much resembles this at times, to the point where one wonders if he's an English Bulldog, as he closely resembles physically, or if he's a Golden Retriever.
Justified in that this was how his mother raised him. After his father, a conman and literal rat left, Jojo Watterson raised Richard to be unable to do anything himself and instill a fear of everything in him, hence why he is the way he is today. We learn the latter when she attempts to do the same with her grandchildren in an episode (but Nicole put a stop to that.)
Sterling Archer in Archer, despite being a world-class secret agent, it also a brainless and immature Momma's Boy who is often more interested in things like pirate lacrosse or Aliens-style space robots than the safety of his companions.
"I must admit, it's sadly anti-climactic. Behind all the sturm und batarangs, you're just a little boy in a playsuit, crying for mommy and daddy! It'd be funny if it weren't so pathetic... Oh, what the heck, I'll laugh anyway! (Mad Laughter)"
Years later the second Batman, Terry McGinnis, throws the Joker's insults right back at him by pointing out that the Joker himself is also this trope as his fixation with the original Batman was childishly pathetic and, once you take away all the garish make up and fancy props, the Joker was really nothing more than a stupid, overgrown kid trying to make people laugh at a joke that wasn't all that funny to begin with, angering the Clown Prince of Crime greatly.
The animated version of Beetlejuice. The majority of the inhabitants of the Neitherworld are fairly mature. Beetlejuice most likely expired in his late 30s, but he's mentally about eight, and a childish prankster who hangs around with a twelve-year-old Lydia in an Intergenerational Friendship.
While he technically isn't this yet, Debbie Douglas expresses hope that her son, Duncan becomes one of these in Freakazoid! because he's not exactly opposed to it.
Debbie: "You'll be a big strong man-child, still lifting weights in your room when you're forty!"
Duncan: "Cool! That way I can really buff up, and save some money!"
Philip J. Fry from Futurama has since the beginning of the series been the epitome of Generation X (or Y perhaps): A big 20-something man-child. He plays video games, watches TV, drinks soda by the gallon, does anything if he's dared to do it, and has no ambition in life except for one... love. He's that flat mate that you knew who just can't seem to truly grow up (...and is often fully aware of it. Refer to his one ambition for why this is a source of gloom for him often), but at least he's cool to hang out with.
The new season reveals that Fry is 35. Adult Child, ho!
Actually, he might be as old as 31 since in one episode, he and the Planet Express crew were affected with chronotons that reversed their aging and then they negated the effects by going into the Fountain of Aging. And they, sans the Professor were reborn in the series's revival.
Fry is his own grandpa and that caused genetic damage, which caused his brain to develop abnormally.
Also, 'cause of that whole Lars is an alternate Fry plot point in Bender's Big Score, this puts Fry's age at just older than 2037 years old.
Also, Zapp Brannigan, though that's more disturbing as he is in a position of power, with the authority to get people killed in mass amounts. Which he fully utilizes to do just that.
The titular character of Johnny Bravo. He's in his mid to late 20s, still lives with his mother, throws temper tantrums when he doesn't get what he wants, spends much of his time playing with toys, has a lot of childlike interests, and he sleeps with a teddy bear and baby blanky often while sucking his thumb, on the other hand he's pretty perverted and constantly hits on women.
Luanne Platter from the later episodes of King of the Hill when she began dating and eventually married a moronic, middle aged, hillbilly "Lucky" to the point where she acts like a five year old most of the time.
Peggy Hill was once described as a woman-child, possibly due to her self-centeredness.
Most of the adults are immature in some way, to the point where Bobby snaps after his grandpa and step-grandma leave him to care for grandpa's infant son and Luanne and Lucky prove to be ridiculously irresponsible parents (I think Luanne is pregnant at this point).
Hank's boss, Buck Strickland, is so childlike to the point where Hank always have to bail him out whatever trouble he gets into.
King Rollo's daily schedule is indistinguishable from that of a five-year-old boy. His kingdom doesn't seem to suffer too much from his behaviour, though.
The Legend of Korra: Despite having grown up an orphan living on the streets, Bolin is pretty immature for his age and circumstance. Justified in that his older brother Mako has done a lot to shelter him from the worst aspects of their lives, and essentially raised him on his own. There's a tinge of deconstruction to Bolin's treatment because it shows the consequences of raising a manchild on the person who shelters them from reality. Mako is emotionally stunted due to having fixated for so long on raising Bolin. His romantic relationships feature him playing the part of a protective figure, and when a girl proves she doesn't need his help he loses interest. They both eventually get better.
Also Aang and Katara's son Bumi although he's a middle aged man he acts like a hyperactive, childish prankster, his sister Kya lampshades this at one point:
Kya: Even though you're the oldest you've always acted like the youngest
While it's not clear how old the Ferocious Beast in Maggie And The Ferocious Beast is, he appears to be relatively physically mature. However, he acts more like a four-year-old, and six-year-old Maggie is far more levelheaded and responsible in comparison.
Toki Wartooth from Metalocalypse. He has a teddy bear, eats candy, collects model airplanes, says "Wowee", and has a childish naivety for certain subjects such as sex. He also drinks, swears, and goes on the occasional murderous rampage.
The main characters of My Little Pony: Friendship Is Magic are all responsible young adults, more or less, setting aside when they need to be relatable to young children. That said:
Pinkie Pie acts like an excitable six year old and seems incapable of holding a serious conversation or actually reading the mood. She can be surprisingly neurotic and insensitive whenever events aren't as exciting as possible or if she isn't getting the kind of attention she wants.
Rarity has drama queen tendencies that are usually Played for Laughs, but, at their worst, they'll cause her to lock herself in a room and refuse to come out like a petulant child and sometimes breaks down and cries with little provocation. Her younger sister can hold it together a lot better than her most of the time.
Princess Luna tries to assert an air of cool authority, but it doesn't take much for the mask to break and this trope to come out. Endlessly enthusiastic about everything, she has a Hair-Trigger Temper and will sulk endlessly if she doesn't get her way. She is desperate for authority, but tends to resent it when it is actually given to her. Appropriately she gets along very well with children.
Ray Stantz, one of The Real Ghostbusters, is a more subdued version of this trope. While mature and perfectly capable of looking after himself, Ray still possesses an almost childlike idealism and enthusiasm for life. Ironically, this actually helps the Ghostbusters lure the Boogieman when they learn that the monster is attracted to people who think like children.
The Warden of Superjail!. Even lampshaded by the Mistress.
Amethyst from Steven Universe. Some of her antics are reminiscent of a teenager acting out. Her yelling "I don't wanna!" when Garnet tells her to go back to the temple in "Tiger Millionaire" is a good demonstration. She's actually quite young in comparison to the other Gems, having been created on Earth instead of hailing from Gemworld. This is further enforced by the fact that during Greg's flashback in "Story for Steven" she's much younger in appearance than she is now.
ThunderCats (1985): At the beginning of the series, Lion-O was placed as a child in a stasis pod to keep him from aging while Jaga drove their spaceship to the third planet. His pod was damaged and functioned imperfectly, so unlike the others he still physically matured inside and he eventually left it as an adult, thus making it a case of Justified Trope.