I was 31 when the attacks happened.
While I do think that there was an awareness that an attack was possible, or even in the works. I sincerely doubt that anyone truly thought that 3 airplanes were going to be flown into buildings on that day and one crash in a PA field. The US had the attitude that we were isolated and well defended enough that such attacks were unthinkable. The complete one sidedness of Gulf War 1 really gave the US an out of proportion notion of being invulnerable. Even though the WTC was bombed 9 years prior, two years after the end of GW1.
Conspiracy denotes malicious intelligent intent. The reality is closer to stupidly complacent. Sometimes the two are hardly indistinguishable.
Lies have a well known conservative bias.