At what age do you speak about sex to kids?

My daughter is 8 and we were reading a book we got from church. It explains stories about the Bible and goes into what is good and bad in the eyes of god and we got to a chapter that starts talking about sex relations should be only in a marriage. Well I just kinda froze because I haven’t spoken to my daughter about sexual relations. Is this a good age to start ? She knows her body parts and knows that no one should touch her etc but that’s about it. what should I be explaining to her ? What age is a good age ? How do I know if I’m over explaining ?