Probably when I was 7, where I saw my father playing some online pool game. Used it when I was 9 when I finally started playing online games and my academic results has sinked every since...
and I was ninja'd.
So, to synerjee's question:
I can't remember, probably when I was 8, went to a local theme park. The queue was extremely long though.
How old were you when you knew about PC?