LMAO!! Some of the answers here!
It varied, from what I can recall.
My first exposure to anything pornographic was a Madonna video. Shocking, I know. It was 'Express yourself'. I remember getting this weird feeling back then and I still blush whenever I see the video. I noticed a similar pattern in other music videos as well. By the time I was 7-8, I pretty much figured it out. I used to watch Animal nature shows so when I put two and two together. I never mentioned it out loud because I didn't want to get in trouble. My first exposure to hardcore porn happened when I walked in on someone watching some pretty heavy stuff. I dont recall seeing male parts but I recall seeing a girl's boobs.
I also remember in third grade, our teacher gave us these animal profiles. We were divided by four to a table. My table had the Siberian tiger. She said 'if you had any questions, ask me'. Well, in the profile, the word 'sex' came up so being mischievous in my own way, I sent someone from our table to ask her. I wanted to see her reaction. I know this sounds a bit sophisticated for someone my age but I swear this happened. The teacher's response to the query was that it was 'male or female'. Our table was DYING because she underestimated us. True story.