The Doctor (10th incarnation) in episode 5 of Ducktalez. When Dewey asks for his advice for the time travel situation, he messes around with his Sonic Screwdriver instead. During the stinger, he pretends his TARDIS is an automobile. He also shows up during the bowling montage in episode 7 and zaps Gizmoduck with his Sonic Screwdriver.
Gordon Freeman, as depicted in Freeman's Mind, was this the even before his mental breakdown, apparently acting immature the whole time he worked at Black Mesa and doing many childish things like playing racket ball in the anti-mass spectrometer and doing a cannonball into his bath tub. And there's the episode where he discovers a cart that goes around in circles at high speeds. He just spends a full minute sitting there pressing the button that sends it back and forth while screaming "Weeeeeeeeee!". Let it be known that there's an alien invasion the middle of all this.
Jimbob in GEOWeasel is an adult man who is interested in such things as Nick Jr, trick-or-treating and waiting for Santa Claus.
SCP-590 was turned into one by the SCP Foundation, specifically ordered by his brother Dr. Bright, by using his ability to heal several people suffering mental retardation until he had the mind of a toddler. This was supposedly done as an act of "kindness" so he wouldn't realize his predicament.
SCP-321 also appears to be this as she's incapable of speaking coherently and is strongly implied to be mentally retarded. This is likely because she was originally stillborn but was revived by her father using several SCP items and that period of brain death damaged her mental faculties.
Talking Classics star Keith Apicary is a big one. He apparently still lives with his family, has No Social Skills, isn't too smart, a klutz, and spends his days playing and singing about video games.
Guy and Ellers both from Chronicle of the Annoying Quest. Though Guy is more cynical while Ellers is a Casanova Wannabe, they both have extensive arguments about Star Trek, Doctor Who and just about anything associated with geekiness, never really taking their quest all that seriously. James, unlike his brother Eric, is naive to the point of stupidity. Goliath frequently asks questions any adult should know the answer to, and exasperates his wife constantly with his immaturity (oh, and he's also responsible for crashing the Exodar). Finally, Stephi is a typical Dumb BlondeValley Girl who thinks Dirim is her boyfriend. Basically a quarter of the series' cast fits this trope in some way.