I was in grade 4 when it happened. We were informed at school of it and it was shocking - watched news later at home. I'd much rather forget about it. Something stunning to see and horrible no doubt. However, there are other things that continuously happen on Earth with massive amounts of loss of human life. I think a reason why it was so shocking (well looking back) was that for arguably the first time ever North America (mainly USA and Canada) had seen something on their doorstep which had the appearance of war.
Fun fact for LX - September 11th is also National Day of Catalunya.