I used to think that once you were past the age of 20, you lost the ability to run, fall down, or sit on the floor. I think this is because I never saw my parents run (come to think of it, I still haven't) and I guess I assumed the reason my mom kept telling me not to run off was because she wasn't able to keep up with me.
A childhood belief about getting older

others believed things like this
- my mother was able to stop me from sucking my thumb by telling me it would rust and fall off!!— lv
- When I was 3 I assumed that my parents were 18 and that every grown-up was 18.Anon
- As an artist, i used to believe that my drawings were visions of my past life.(i had a good past life)— Blind dreamer
- I used to believe that aliens would come and take me if I wasn't good to my parents— somebody
- I thought if you trip and fall down you fall at the speed of light!— Dietrich Corby
- I thought 'infrared' was the past of 'to infrare'.
— Lloyd B