I used to believe that your reflections were YOU in the mirror world, and at age 17 they could choose to be your reflection or go off and do something else. This also let to the assumption that vampires were people who mistreated their reflections, so the reflections deserted them and made them vampires. Then I thought Dracula was the meanest vampire of them all, since he scared his reflection away and became the most widely-known vampire.
A childhood belief about vampires

others believed things like this
- i always thought I was the reflection and your reflection was a personAnon
- I used to believe that when you cracked a mirror your reflection would come and take over your body!— XxXAbbeyXxX
- I used to believe that my reflection in the mirror was my evil twin that always had bad hair. XDAnon
- If you stare into a mirror in almost complete darkness, the devil will appear instead of your reflection.Anon
- I used to believe that the people who take your blood at the doctor's office were vampires. — Andy
- I used to think I could sneak up on my reflection and catch myself not looking back at me.— Mary Poppins