I was 20 years old and at my internship. We had TVs along the hallway in the center of the building. I remember looking up (I could see the TVs from my cube), seeing a number of older coworkers watching the news after the first plane had hit, I got up to join them. Shortly after I joined them, the second plane hit, and I remember muttering "well, we're at war...". A colleague roughly 3 times my age and close to retirement looked at me and asked, "what do you mean?" Response was something along the likes of "well, one could be an accident, but two? No accident, that's intentional." Of course, we later heard about the other 2 planes.
I was supposed to go through downtown Chicago that day to turn in a project at my college campus, but I politely emailed my instructor, telling him in no uncertain terms that there was no way in hell I was getting anywhere near downtown that day. I emailed him my project and told him I'd turn in the physical copy when things were safe. At the time, I was living near Ohare. It was eerie how quite it was without all of the jets taking off and landing. When you did see planes, it was a pair of either F-15s or F-16s running combat air patrols. Very surreal experience.