My girlfriend and I were watching Portlandia today and they asked this question to one of the characters and it got me curious as to where people where when you first heard about the plane crashes? It's interesting that most people can't recall what they did on their 18th birthday, but everyone seems to remember where they were when they first heard about the crashes.
I was in 4th grade at the time and I was meeting up with my neighbors to walk to elementary school. My neighbor came out of her house and asked if I had heard about the planes crashing into the Twin Towers (being a 4th grader, I had no idea what they were to begin with). It was cloudy that morning and my neighbor thought that all the clouds may have been caused by the smoke of the crash.
I was asleep. I was born in 1994, so when 9/11 occurred I was in the first grade. I woke up because my older brother woke me up, and I remember going down stairs and seeing our family watching the news on the TV in the living room. I then remember being in school and talking to lots of other people about it, mainly asking if they'd heard.
I have to say that I really didn't realize what was going on. I was 7 years old, so I hardly could understand what was happening. It's odd looking back on that.