When did you discover sex...?
... as you understand it today?
How about other intimate things? A naked body? A bared breast or a penis?
What did you think you knew before that?
What happened in your childhood to clue you? If nothing, do you now see things your parents did to hide private things?
How about older siblings?
Friends?
For example, looking back, I know that my mother was very careful about covering herself, and my dad was less so, although he tried most of the time to have underwear on.
My parents watched grown-up movies while I'd be playing somewhere else, but I was pretty good at sneaking up on people, so I remember some things that I saw that confused me, but not creepily so.
I had two boys as best friends when I was little, and so obviously at that age sometimes there were "accidents," or i'd go into the bathroom when a friend had to pee. He wasn't quite tall enough yet for the toilet, so his mom ended up helping out.
We were given the body part lecture in fifth grade, and the physical action bits in seventh but ultimately eighth grades.
I remember being in fifth grade and asking someone "does it actually go in?! and being answered yet, and that was quite an alarming revelation but everything that came after that was okay, I never got a nasty shock by any other information, no matter who gave it to me.
I started puberty very very late, at 17, because I had had some chemotherapy, there was some question if I'd ever go through it.
And yes this story is believable because I was raised in Nebraska and so had a fairly sheltered upbringing.