Going back on Topic - I think that the earliest kids should be told about it is roughly 5-6th grade (say around 11 years old or so) but in broad terms - there was a terror attack by some bad people. They wanted to kill or hurt americans, a lot of people were killed. Obviously they may still express anxiety and fear, but it's probably easier for them to comprehend and have their fears assuaged than a 7 or 8 year old. (speaking from parental experience - I am not a shrink).
I don't think that the details need to be shared with kids below HS. i.e. the 'misah meshuna' discussions that we had on the original thread are not for kids below 15 or 16. (ex. People jumping to their deaths to prevent from being burned.)