It can be from any time in your life, not just when you were little (though those tend to be the most entertaining).
I thought astronauts were a different species. I thought they collected things in space, and that inside the suits they were just blackness.
The first time I saw a garbage truck take our garbage, I thought they were stealing it. I yelled for my parents, "THEY'RE STEALING OUR TRASH!"
I thought "Irish need not apply" meant that if you were Irish, you didn't need to fill out an application and would automatically be accepted on account of being Irish.