I belived that beard and mustache were roots of hair. That it took long for the roots to grow out from ones chin and that's why only adults had them.
A childhood belief about hair

others believed things like this
- I used to believe that rootbeer was made from the roots of a beer plant.Anon
- When I was little, I couldn't wait to grow up so I could grow a mustache. I am female.— Laura
- I used to think a mustache was just extended nose hair.— Dan
- I used to think a mustache grew out of your nose hairs. I tried to let mine grow, but it didn't work. — Barry
- When i was young about 5 i used to think god was a donkey with a long beard!Anon
- How could Baby Jesus be born on Dec 25th, get tall, grow a beard and then die, all by Easter. Whew!— Joe