This is a question that some people (when I used to read alt.society.generation-x) used as a marker for belonging to Generation X; if you were too young to remember the explosion, then you were a Millenial. It was an equivalent to the Baby Boomers' "Where were you when you heard John F. Kennedy had been assassinated?"

Me, I was home sick from seventh grade, watching MTV in my bedroom; my mom came in from the living room to tell me what had been announced on the station she was watching. I remember MTV stopped using their "moon landing" station identification for a while, feeling that any reference to astronauts was inappropriate at the time.