No particular time. 9/11 is in the history books now. Kids learn about it the same way we learned about Vietnam and JFK, or the way kids slightly younger than me (40-ish) learned about the Challenger explosion. There is a certain level of detachment because they didn't live it firsthand. My DS15 was one year old when 9/11 happened, obviously with no memory whatsoever of it. DD11 and DS9 were not born yet. They have all learned about it in school, and I have talked to them about my firsthand experience with it. They have seen the documentaries, books, and movies. It affects them as much as learning about the Nazi's in WWII or the Union/Confederate soldiers in the Civil War.
They are much, much more affected by the Boston Bombing, and especially by the Sandy Hook shootings. DS9 was the same age/grade as those little 1st graders. DS15 was in Middle School, and DD was in 3rd grade. It was all explained to them once they had to start doing "intruder" drills at school as a result of that tragedy.