My parents never liked to celebrated their own birthdays and I assumed that only children's birthday parties had candies and cake. So I assumed that grown-ups didn't grow up anymore because they didn't eat sweets on their birthdays while kids ate cakes and everything and grew taller/bigger overnight.
A childhood belief about birthdays

others believed things like this
- Ironically my parents were born on the same day; so growing up, I thought all parents had the same birthdays. Anon
- When I was 3 I assumed that my parents were 18 and that every grown-up was 18.Anon
- I used to believe that on your birthday, you didn't advance to your next age until you ate your birthday cake.— Casi
- Migets were people who were bad when they were kids and there parents didn't let them grow up.— Teresa
- i thought that when it was your birthday you grow and then you didn't grow until a years time.— hello
- i used to believe that you were suppossed to grow taller than your parents, and then when i didnt i was sad.— crysb83