I knew about sex when I was 4, before I'd even started playschool. Seriously. I religiously "read" anything I could get my hands on, and being mischievous I decided to root through a ton of medical books my parents had stacked away in a cupboard. It took my mum a few seconds to figure out what had happened when I pointed at my neighbours kissing across the road and exclaimed "they're going to go inside and rub bellies and have babies!". She had to sit me down and tell me not to talk about it with my brother (who was 6 at the time) as it was something very special and secret that only adults did with each other.
I (admittedly vaguely) remember taking it as fact and just another thing that happened in life. I had a harder time digesting the outrageous news that certain seasonal characters I admired did not exist.
I don't believe it makes a difference what age children learn about sex. Some will be ready for it, some won't. I say if a child makes statements and asks questions it is up to the guardians to either correct them or inform them. Not only does it mean they become more knowledgable and perceptive at a younger age, it also teaches them valuable lessons about life that will stick with them indefinitely, preferably before they make blundering mistakes or talk about the wrong things with the wrong people.