I used to think there were FOUR genders: Boys, girls, 'wrinkly boys', and 'wrinkly girls'. I was growing up with my grandmother as a third parent, but I was convinced she wasn't the same gender as me and my mom; how could a girl's skin get so wrinkly like a...a wrinkly girl's skin!?
On a similar note, I used to think height was the end-all indicator for age, so for a while I thought my dad was older than my grandmother (his mom). Years later, I actually believed that once you reached a certain age, you would shrink until you died from being old.
A childhood belief about getting older

others believed things like this
- I used to believe that old people were wrinkly because they spent too much of their life swimming— Amy
- When I was a little girl, I believed that all cats were girls and all dogs were boys. :)— Mary Ellen
- I thought that the difference between girls and boys was that girls have teeth.
Anon - When I was younder, I was convinced that all dogs were boys and all cats were girls.— Clare, Ireland
- I used to believe that numbers had gender, that the odd numbers were boys and even numbers were girls. — Evelyn
- When I was little, I believed that all cats were girls and all dogs were boys. Go figure!— Laurie W.