I think I was 9. I used to "go out to play" with the teenage (14-ish I think) girl over the road who, naturally, was obsessed with talking about boys and sex. She told me what sex actually was (and a few other things besides), but whilst I then knew the mechanics, didn't really understand any sense of context or the implications. I then had a particularly silly day (probably eaten too many E-numbers) when I saw a board game in the Argos catalogue called "Battle of the Sexes". Much to my Mum's embarrassment, I started giggling and going on about "hahahaha, sex! I know what sex is! hahahaha!" etc like the annoying brat I was. That evening, she sat me down and gave me "The Talk".
We've both been scarred for life since.