I used to believe that boys were always older than girls. Possibly, because men are often older than women in relationships. Also, my eldest sibling was a boy.
A childhood belief about appearance

others believed things like this
- a friend of my little sister's had 4 older brothers, so she thought that when you got older you grew a penis!Anon
- when i was really young, i couldn't figure out why i can't get older than my older sister.— angela
- That boys turned into women when they became adults and girls turned into men when they became adults. Anon
- I thought that Dad's were always older then Mom's, come to find out my mom is a few months older than my dad.— Nicole
- I thought my grandad would get older, so I asked him 'What do you want to be when you're older?'.— Afterlife Dreamin'
- I thought that the difference between girls and boys was that girls have teeth.
Anon