I remember perfectly.
I was 9. Me and my friend were sitting at home, in the kitchen. He went there to for we to trade and have some Pokemon matches (I won them all ). The TV was on, although at the time I wasn't watching, and suddenly, news flash! A plane crashed into one of the Twin Towers. At the time, we were giving a crap about all that. Barely we knew the impact it would have and how much it would be talked about.
I also remember, later in the afternoon, the news host was talking about the first crash, and appeared some footage. At first, he thought it was the footage of the first crash, but then he realized it was live footage, a second airplane was crashing against the other tower. It was a surprising moment.