I used to think that adults never cry.I think this might be common because when you were a kid and the adults would say "oh don't cry big girls/boys don't cry".
A childhood belief about getting older

others believed things like this
- That boys turned into women when they became adults and girls turned into men when they became adults. Anon
- That adults came into the world as adults and were never childrenAnon
- I used to think that I was smarter than adults. Now I know kids are smarter than adults in many ways— Marshall M
- i used to think that adults did not have dreams!Anon
- I used to to believe that all adults went to bed at midnight.Anon
- I used to think that all adults were robotsAnon