People agonize about when to tell the truth. Many people feel their kids will loose their innocence if they know about it. I've even met mums who have cried as their child learned words like penis and vagina at the age of 9( from school not them.)
People are scared to talk about it.
When dd asked at 3 I told her an ave appropriate version of egg and sperm then built on it a bit since she got older.
I'm not the perfect mum when it comes to this but I do wonder why we all get in a flap about the facts of life when it is vital for kid's safety ( and scientific fact.)