What is a Kidult???
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...
A grown up who is also a kid. A fun loving, nice, hyper, funny, slightly inappropriate person. And super, super weird.
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".