I was 21. Worked the night shift at the time and school during the day. Woke up earlier than normal for breakfast with the infant, made coffee, and turned on the news. Saw the first building burning and at that point, they didn't know if it was accidental or intentional. I kept thinking "how big of an idiot do you have to be to hit that building." Then as I was watching, from the side of the screen the second plane hit and the explosion. I was in complete disbelief. Looked like a movie. That's when they said something along the lines of "we're under attack." Fucking wild day for sure. Life as we knew it as Americans changed drastically that day.
Edit: Just to add to this. I've also, in my 40 years, never seen 100% of Americans come together like that in a show of patriotism. At least for a small while. It was our modern-day Pearl Harbor. Flags flying, you gave a shit for once about your neighbor, etc. Wish that would've stayed.