When I was a child, I met my Nana, my grandmother's mother. She was 86 at the time, and reasonably frail. As I grew up, she became frailer, and suffered the sort of hunched-shoulder back often seen in very elderly people, so she lost height.
I remember reasoning to myself that people grew bigger, and then as they got older they grew smaller again. I believed that for quite a while. If the process continued, I thought she would become as small as I was eventually.
A childhood belief about getting older

others believed things like this
- I thought plants only grew when people were not looking at them.Anon
- I used to think wrinkles came about because people just grew too much skin.Anon
- I used to believe that people only grew, physically and in age, on their birthday. — Aretia
- a friend of my little sister's had 4 older brothers, so she thought that when you got older you grew a penis!Anon
- I used to think bread grew on trees and people just cut it up and sold it.Anon
- I used to believe people grew from spores and when we got old, we'd grow leaves and flowers etc— Calvin (from my pet tiger Hobbes)