Okay, I have to admit that I really don't have any clue where this belief came from, but when I was much younger I thought that I would eventually not only grow bigger than my older sister, but that I could/would grow older than her too.
A childhood belief about getting older

others believed things like this
- My older brother used to tell me if you ate raw potatoes your teeth would grow long and curly!— Lauren
- a friend of my little sister's had 4 older brothers, so she thought that when you got older you grew a penis!Anon
- I used to belief sloths lived in trees outside my childhood home. My older sister told me so.Anon
- When I was a child, I used to think that if I lied my nose would grow bigger.Anon
- when i was really young, i couldn't figure out why i can't get older than my older sister.— angela
- I used to believe that everybody who was bigger or taller than I was always older than me.— Stano Finji