A so-called grown-up who doesn't want to grow up (or at least act like an adult) and would instead prefer so-called "children's" stuff for entertainment, like cartoons, toys, comic books, Disney movies, etc. He or she also enjoys colorful "kiddie" snacks like breakfast cereal and Spaghetti-O's and dresses like a teenager or perhaps younger. May or may not be great parents as well as being able to take on adult responsibilities. Not necessarily too immature at least in the public. Not to to be confused with geeks or nerds either, of course!
Also known as "Peter Pan", "young at heart", and "kid at heart".
I'm almost 35 and I love Neopets, Archie comics, children's books, etc. So this makes me a kidult, heh.