There in know right age. You should talk to your kid about sex things as they ask. Kids ask crazy thing as soon as they can talk good. "Why doesn't daddy have breast like you mommy". Theings like that. Or they see a male naked or a female naked. They start asking a lot of questions then about body parts. Or how do women get pregnant, that one comming out of a 5 year olds mouth. By the time they are teens if you talked to them as they asked. They should know a lot about sexual stuff. Then you will just have to alk to them about STD's if they haven't asked.
I never had the talk with any of my children. Instead, I answered all their questions as they came up, and we also used life moments to teach. My children are now 40, 37, and 33 and they have all been very well adjusted people without having had the "talk".
I agree with them- there should be ongoing discussion of different things - use tv, music, whatever as places to start and just let them know you are there for them no matter what and don't get freaked out when they ask a question, and they will will know they can come to you