So, what age do you talk to your kids about what happened on 9/11?
IF and when they ask.
Whenever they learn about it in school
Are you saying never? I honestly don't think any of my kids schools would teach anything about it, nor do I see a need for any Jewish school or Yeshiva to specifically teach anything about it.
BTW, how many people 'round here know it also as כג אלול without looking it up?