I was about 10 when it happened. I remember my parents showing me stuff about it and we talked about it in school. I remember being sad but not fully realizing what happened. Years later I'd stop caring and just laugh at 9/11 memes. If anything, it's amazing in hindsight that in 2002/2003 we glorified going to war with Iraq and the other rushed responses to it. (ie the Patriot Act) My little brother was 8 when we declared war and he brought home some Scholastic news magazine that usually had educational stuff glorifying the patriots going to defend our freedom in Iraq. Wish we kept that just to show how poorly it aged.
I hate the fixation on 9/11 and "Never Forget". Yeah, it sucks people died, but you can't spend 20+ years grieving and letting politicians pass shitty laws to take advantage of the fear. I'm more curious if the older generations spent this much time grieving over Pearl Harbor. 9/11 needs to be treated the same at this point. Both were a depressing moments in history, but like Pearl Harbor in the present, 9/11 needs to be accepted that it happened without bogging down our daily lives in remembrance.
As far as Zoomers, none of the older ones I've talked to remember anything pre-9/11. At best their first memories were right around when it happened or of their griefance/confusion. Unfortunately it defined the later half of my childhood, but I still remember everything just being "normal" or less afraid before then. I even remember picking up family from the airport right outside their terminal before security revamped and only let passengers past security with minimal exemptions. (escorting the disabled, children flying alone, etc) I've even been in the cockpit of the plane and talked to pilots a few times. (I forget if the plane was flying or landed, but regardless good luck doing that now.) I don't know if Zoomers can even comprehend doing anything like that anymore due to the increase of security and surveillance that they grew up under.