I just figured it out when I was seven. I still remember the moment of realization. It didn't hurt, though, that I had absolutely no misconceptions about anatomy because I had two younger brothers at the time, and I often bathed with them + we'd run around the house naked (though I was younger than seven when I stopped doing that lolol) because that's what little kids do. All my existent cousins but one were boys at that point, too, and the ones I played with and I are all within four years of each other in age, so, you know, that was never a mystery for me.
I did confirm my suspicions, though, by looking up "sexual intercourse" in the dictionary, just to be sure.

When I was eight, my mother came into my room one night as I was going to bed and awkwardly tried to explain the basics. She used the correct terminology, penis and vagina and all of that. She explained periods and what their function is and blah blah blah. I was like, "I already know this," but I just let her talk to get it over with.
What I end up doing with my own children depends on how perceptive they end up being and how close they are in age, what genders they are, etc. Got a pretty perceptive kid with a sibling of the opposite sex within two years of age and aren't a paranoid weirdo about nudity? Yeah, they'll probably figure it out fairly young. I still plan on talking about it, though, around the age of seven or eight, just to explain how sex leads to babies. If they don't know what sex is by that point, I'm sure they'll be curious enough to ask because children really have no embarrassment when it comes to that the way they suddenly do when they hit about age ten. If they already know, they won't ask, and I won't need to make it awkward for them.

I'll know they have it down for sure when I have eleven and twelve-year-old boys that turn into little pervs.
