They don't know a pre-9/11 world.
Sometimes this thought stops me in my tracks. The majority of my life was a pre-9/11 life. I met my husband, moved out of my parents' house and became an adult in a pre-9/11 world.
I can't even imagine a post-9/11 childhood and yet that is the hand dealt to my children. I am a post 9/11 mom with post-9/11 children. That date, for them, will be what JFK's assassination or Neil Armstrong's landing on the moon was for me- not even a distant memory, but rather the memory of a generation before. They will read about it, they will hear people talk about it, but it won't be real to them.
They will never understand how the world changed that day.