I think parents should talk to their children about important family and life values throughout their entire life regardless of religion. It builds trust and friendship between the child and parent.
When it comes to talking to them about sex it should be not according to their age but maturity.
Some children do not understand or cannot picture this in their mind emotionally, whereas other children understand at an early or appropriate age.
The importance of talking to any child is to make them feel comfortable about this most delicate subject, and most of all to be a good listener.
Both parent/guardian and the child should feel open and know sex is normal, it's special, and it's not to be used over and over again promiscuously, but in a respectful way.
This will not only teach them to respect their partner, but most importantly to respect themselves and take responsibility.
Children hear so much at school and with friends they meet and hang out with, that it can be not only overwhelming and scary, but if you don't talk to your children as their confidante - they could very well be pressured into having sex to soon or unwillingly just to "fit in with the crowd".
I think if you wait too long, your child will hear if from another source that may be outside your family values or morals, and then it may be too late.
If you want to check out an excellent book to help you, turn to the authors Kline and Faye.
They master this subject for teens as well as other important life skills.
If you as a parent are asking this question online - maybe you're feeling it's time.
If you're still not sure - ask a school counselor, teacher, family doctor or check out books at your local library.
Good luck. Good question!