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.
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 and bat-o-rangs, 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.
Dan Vs.: The titular character as well as best-friend Chris are definitely this. They both watch cartoons, read comics, and recently argued over sci-fi memorabilia. Though Dan is arguably worse, having no job and no girlfriend. Chris often depends financially on his wife and wanted to give a woman a mechanized dinosaur doll as an engagement present.
Chris' wife Elise finds Chris' childishness endearing, and seems to switch between endearment and irritation with Dan. It's gotten to the point where she sometimes treats him as if he were their kid. In "Dan Vs George Washington" she lets him sleep with them (because he couldn't sleep) and proceeds to tell him a bedtime story, much to her husband's disdain.
Dan "Tell me a story..."
Elise "There was once a little prince who was so very angry-"
Dan "Was his name Dan?"
In that particular instance, Dan couldn't sleep because he was upset that his plan failed, and he says it in a very childish way, sounding more like a kid with a drawing no one wants to look at than a man on a vengeful rampage.
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.
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.
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.
Texas and Tooley in Motorcity. When the former is telling the latter about "his" exploits, Tooley even lies down like a kid would.
The main characters of My Little Pony: Friendship Is Magic are all responsible young adults (with the possible exception of Pinkie Pie). Their behavior, on the other hand, often ranges from emotionally immature (generally, to set up a lesson in friendship) to downright childish. For example, take the following bon mot from cosmopolitan fashion designer and social climber Rarity, upon hearing that a game of Pin the Tail on the Pony is starting: "Oh, my favorite game! Can I go first? Can I have the purple tail?"
In all fairness, the characters are ponies. Also taking potential culture into account, they may have different concepts of aging and maturity.
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.
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!"
Amethyst from Steven Universe often comes off as this. She's lazy and immature.
Thundercats: At the beginning of the series, Lyon-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.
Xiaolin Showdown: Kimiko's Dad is a child on the inside. Or so she once told her friends.
As with the comic example above, Captain Marvel in Young Justice is really a kid. However, none of the teens know his real form, so he definitely comes off as being this trope.