It kind of blows my mind that there are people living now who haven't lived in two different millennia like me. My earliest memory is from 1999 of my brother being born. It's kind of weird to think that it was literally a different millennium, and that I (and most people living right now) am one of the really very few people in all of human history who get to experience this.
Until you realize that millennia are just arbitrary groupings of 1000 years.