I believed that when you get older, you change names. That's prolly because in our language some names are now rarely used (I thought they're old man names, since I didn't know any kid with such name) and the currently common ones are "kid ones". For example, when Billy grows to John's age, he will be renamed to John and John will be renamed to something else. Since Billy sounds "childish".
A childhood belief about names

others believed things like this
- As a child I used to think that Olivia Newton-John and Elton John were married.
Boy, I was way off.— Lisa - As a child, I thought that Olivia Newton-John was Elton John's wife and Juice Newton's sister.— Connie
- I thought that Olivia Newton John was a group, Olivia Newt and John. — Sam O.
- I used to think that Olivia-Newton John was three people like Sharon, Lois and Bram....Olivia, Newt and John— J
- I used to think that Olivia Newton-John was 3 people: Olivia, Newt, and John— Sally
- Because of MAD Magazine, I believed that Elvis died while on the john, with a tub of KFC in his lap.Anon