I think I believed that on your birthday you didn't turn a year older until your birthday party but this was disproved on my fifth birthday and my mom was rambling about me being five but I said "No I'm still four" because my party wasn't until later that night and my mom said "No you're five."
A childhood belief about birthdays

others believed things like this
- My parents have the same birthday -- month, day, and year. I though all mom's and dad's had the same birthday.— Cait
- I used to believe that on your birthday, you didn't advance to your next age until you ate your birthday cake.— Casi
- 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 thought I got one year older when my family sang the birthday song.— I am the ghost
- My sister uesd to think that you did a whole year's worth of growing in the night before your birthday.— Richard Davies