I used to believe that people changed completely as they got older.
First, I would begin to hate cartoons, comics, toys, video games, etc. because I didn't know of any adults who liked them. The process would take about a week, during which my childish interests would be replaced by boys, make-up, clothing, shopping, etc. I would also develop a heavy valley-girl accent and, because of puberty, I would act very air-headed until my hormones had settled. (Thanks, cartoons.)
Later on I would become very dull and work-oriented, but I would be organized, neat, and well-mannered. Still, I was not looking forward to finding pleasure in political talk shows and gardening. (Though, for some reason, I really wanted to learn how to sew.)
About when I was 14 I realized this was absolutely not the case. Unfortunately, that also meant that people had to LEARN how to become orderly and mature.
A childhood belief about getting older

others believed things like this
- I used to think that "cross-eyed" meant that people had X's for eyes, like dead people in cartoons.— Tim
- I used to wonder why people didn't have outlines like they did in cartoons.— Anna
- When I was a child I used to believe that when I played video games my real world changed like the game. — J
- I thought cartoons were for kids and "real-life programs" for adults.— Ichigo
- I used to believe cartoons were real people and animals. They were just wet.— Wendy
- I used to believe cartoons where real people. You can imagine how that went... — Emma