Many or most adults in the U.S. operate at the level of what we expect of children, but not what we expect of adults.

I only specify the U.S. because it's what I know directly.

(I suspect European adults overall are better socialized to behave in an adult way, but are not necessarily actually any wiser than American adults. Just on average. There is an enormous variation from person to person.)

edit: I feel like this needs an example. I'm not saying American adults are incompetent at their jobs, for example. I'm saying they hold childish beliefs, like believing in Santa Claus, but not that one in particular. In other words, they lack intellectual hygiene. Another good example is what came up earlier in the thread: people rationalizing their bad actions so that they are not bad in their own "story."