One of the less arbitrary ways to divide generations that I've seen is the memory of a defining moment. That is, most or all of a generation will be able to answer in great detail the question "Where were you when you heard about Event X?" For my grandmother's generation, Event X was the bombing of Pearl Harbor. For my mother's generation, it was when JFK was shot. For mine, it was the Challenger explosion.
I remember it vividly. We'd all been told about it in great detail, because a New Hampshire school teacher, Christa McAuliffe, was aboard. I don't know if it was being taught and watched in schools all over the country, or just in New England because she was local. I do know there were a lot of schoolchildren watching live. I guess the launch must have been scheduled during the first and second grade lunch period, because they were in the cafeteria with the big TV on. I was eight years old, in the third grade. I was taking a note down to the kindergarten wing. When I walked past the cafeteria, it was quiet. Too quiet. That's a phrase used so often it's cliche, but that was the first time I'd ever had the experience, and it's that uncanny hush that I think of when someone says it. I can't remember now whether I actually went in to find out what happened, which is what I think I did, or whether I just finished the errand and heard about it a little later.
I really can't imagine anyone my age forgetting it. For many of us, it was only our second contact with death (the first being when Mr. Hooper died on Sesame Street – don't laugh if you don't remember that, read about it). And it was the end of most of us having any thought of growing up to be an astronaut. It wasn't so much that we were scarred, as that the space program just stopped existing for the rest of our childhood. And yet, when I had this conversation with my family, I discovered that my brother didn't remember. He was five, a year too young to have been in that cafeteria watching. He remembers hearing stories, but he doesn't have any personal recollection. It turns out that somewhere around the beginning of 1980 really is the generation break based on that event.