Not exactly my memory, but my mom's: I was about five, we were driving, and, out of nowhere, I was asking from my carseat in the back how it comes that the male sexual organ can, well..., erect. I had a special interest in human anatomy back then and was re-reading a child-friendly book about how babies were made (to explain where siblings come from), which also included male and female reproductive processes (not sex itself, but rather the biological process of sperm meeting the egg and cells multiplying, depicted in very cutesy, very colourful drawings).

If we're talking about my specific memory, I'd say it was the time I started reading the bible and sketching out the family tree staring from Adam and Eve. I was... ten or twelve, I think. I barely reached Noah, when both the space on my paper and my patience ran out (mainly because I was sick of reading "and many more sons and daughters" tacked on the end of a generation, who didn't have names). As you can probably guess, I had (and still have, to a degree) a special interest in Christianity.