Sex Ed - 4 year old

I saw a post about age appropriate sex Ed and people's opinions, so thought to ask ya'll this.

So my son goes to nursery/kindergarden/preschool whatever you guys call it (his just turned 4) and yesterday in the bath he took one of his balls and inspected it, I asked him what his doing and he said "i have babies in there", I told him his right but not yet, he will have something called sperm growing in there when his much much older, he replied "sperm when I'm older?", I said yeah, that helps make babies. That's it, he was happy to learn something new, I then asked where he learnt that from, he said school, but not from the teachers, one of his friends told him.

So seems my son is learning things about his body from other peers in his class, maybe their parents are having talks or watched a video or something?

I didn't think I'd have to do it this early but it's fine isn't it?

Do ya'll bring topics up to your kids about their bodies or wait till they ask till a certain age?