For me, conversations started at 11 and progressed from there. If its something that I think they shouldn't have known at that time, I just tell them that & why. I didn't want them listening to their friends or getting some wrong information.
Growing up my parents wouldn't talk about anything (but I sure as hell saw it - seems like I walked in on everyone having sex). I remember one time at 13 I was playing with a boy down the street and my father became angry. They took me to the doctor and claimed that I almost got pregnant
But I knew better because we hadn't done anything. I don't want them to be uptight about sex.