I believed that there was an actual number inside you, somewhere near your stomach, that would change on your birthday. I swear I could feel it "changing" when I woke up on my birthday.
A childhood belief about birthdays

others believed things like this
- I believed we were little kids till we were 17, and at our 18th birthday we woke up as adults.— Hoppip
- Up until I was almost 11, I thought that a birthday suit was a special outfit you wore only on your birthdayAnon
- I used to think that a birthday suit was a special outfit you wear on your birthday— Bri
- I used to believe that on your birthday, you didn't advance to your next age until you ate your birthday cake.— Casi
- My parents have the same birthday -- month, day, and year. I though all mom's and dad's had the same birthday.— Cait
- i could swear eleventeen was a number for the longest time -- it still sounds pretty convincing.— erin