I don't think it should be one conversation - too overwhelming - and I don't think any age is too young. By which I mean, I think it's best to add in more information gradually as they grow older. And if they weren't asking questions by age 6, I'd be volunteering information.
Thinking back, dd1 was asking by 4 where babies came from, and I would tell her about growing in tummies etc - she also had books about the human body that included reproduction in an age-appropriate way.
By 6, she knew about periods (she had spotted tampons in the bathroom). And about male and female seeds.
By 8, she knew about the basics of sex.
She is now 9, and beginning to show a real interest in HOW people have sex, and in how sperm gets out of willies. So we're talking about that, and she has a book that explains it too.
dd2 (5) thinks it's all disgusting and is determined to adopt. I discuss it with her less, because she's not so interested. But I'm confident that she hears it's an open topic for conversation, and there are books around should she be interested.