I was 6.
(Some background information: My stepmom and I would get up early during the weekdays and stop at Tim Hortons and then head over to her friend's house to eat breakfast and have coffee, and then her friend's son and I would go to school together from there.)
Anyways, I was getting ready for school... it must have been about 6am here. My dad and stepmom had the news on, and I just remember hearing, "Oh. My. GOD" come from the living room. I went into the living room and looked at the TV and I just saw a building completely engulfed in smoke and flames. I didn't understand the significance of the buildings, I didn't know what the World Trade Centre was, I didn't know what terrorism was... I just knew that whatever I was looking at, it was BAD. At the time, the adults just described it to me as, "Bad people took over some planes and crashed them into buildings." I also remember it being busy as all hell in Vancouver because a lot of planes were being re-directed to Canadian airports.
I remember seeing the newspaper cover images of the planes crashing into the towers, the emergency services outside the pentagon, the people falling from the buildings, etc. and one particular day after looking at the pictures in the newspaper, I asked my mom why it happened. I'll never forget the look she gave me when she said, "I don't know."
I think 9/11 was my first real, hard glimpse into how terrifyingly ugly humanity/the world could really be.