Just noticed that topic was here, so I wanted to share my own memory, if a little late.
I was twelve years old at the time. I know my father had moved to his own place at that point, because I was having my week with him when my mother called his landline to tell me about the planes hitting the towers. From hearing her reminisce about it in more recent years, I know that thanks to her workplace's ties to the US, she and colleagues watched at least one of the crashes going on live. Between my young age and my autistic spectrum induced social obliviousness, I didn't understand it wasn't some sort of accident at the time of the phone call and may have needed to watch the news footage or something for it to really kick in. I had gotten a quite large drool-proof stuffed Hello Kitty at some "sell your old stuff" event not that long before that and I remember hugging it a lot while watching news footage. I remember a scare around one of my mother's colleagues getting an Amazon package delivered at their workplace not long after the attacks, when people were still legitimately worried that foreign offices of US companies might get targeted by terrorist acts. My junior high school had a music class in which we had been learning the French version of "The Ballad of Jesse James", and I came up with a parody involving Osama Bin Laden. I also remember someone telling me a joke about George Bush always losing at chess (rooks are called "towers" in French, it's all you need to know). A few years later, I was beginning one of my high school years and we were all getting our text books. The one for history had two photos on it, one was of the twin towers sometime between the first hit and their collapse.
When I was a kid, my mother managed to bring me with her to the US about once every couple of years. The last time we were there before 9/11, we happened to be in New York. At some point, we were on a boat that was in a place that had a good view of part of the skyline, including the twin towers. One of my mother's memories from that day is acting on an impulse to point the twin towers out to me that seemingly came out of nowhere.