The earliest I remember is Barbie having 'naked parties' with Ken. But this was before kindergarten to my knowledge of sex wasn't up to snuff. I remember being a little under ten and watching a girl the same age as me sleeping at our house on the couch with her sister. Both of 'em, just flopped on top of each other snoozin' like a pair of angels. That got me thinking. Not much in that line, though, until I picked up hentai which was when I was like 13 or something.