When you get married and start having kids, at what age is it generally safe and appropriate for christian parents to talk to their kids and educate them about sex (aka "the birds & the bees")? How should a christian parent pursue the subject of sex to their children? What should a christian do when their child asks "where do babies come from"?
Rather than being able to reference you to a specific Scripture that talks about when to have "the talk", there are a set of Biblical principles that you can think and pray through with respect to address issues of purity with your future kids.
First, with the prominence of messages of impurity flooding kids these days, there is rarely a specific moment where a child is first exposed to sexual messages at an age appropriate to address it. So, I would encourage you to think less about a single "birds and bees talk" and more about establishing a foundation of understand God's plan for love, marriage, commitment, and at the appropriate time, sex. Building this foundation will take years, but with diligence and consistency, bathed in prayer, it will make the transition to understanding sexual purity much easier when your kids reach the age where they can understand it. Here are some principles and themes that Laura and I are trying to establish with our kids. Some are adapted from Focus on the Family which has some great resources for parenting from ages 0-up, at their website, www.family.org.
God made your body: Your body is a gift from God. You are unique - there is no one like you. Appreciate your body and take care of it. We eat good food to keep it healthy, we exercise to stay fit, we understand the dangers of alcohol and drugs, and we keep our mind pure by guarding what we watch and what we say. As a result we guard not only what we watch on TV and look at on the internet, but also monitor and limit what our kids are watching and looking at. If we don't do, no one else will and the world will have its way with your kids.
Babies come from God - they are the result of marriage and love: This is why we protect their life right from the beginning. Even though God's plan is for a baby to be born with a mother and father who are married, sadly this does not always happen. But marriage is God’s best idea to create a family.
Your body is special and private: No one has the right to touch your body but you. We allow the doctor to touch your body in order to take care of your health, or Mommy and Daddy may need to help you with your body for health, but no one else is to touch it. If someone tries to touch your body or asks to touch your body, you immediately run and tell your parents or other trusted adult. There is no such thing as keeping that kind of secret from your parents. Mommy and Daddy will always be here to help you. Because your body is special and a gift from God, we protect it in how we dress and act.
Commitments, like our word to you and like our marriage, is our bond and should not be broken: When children learn the value of keeping a promise, it helps them keep their word as adults and their promise of commitment when they marry. Little things like completing homework, or practicing for a team or event they are involved with all build character qualities that help kids focus on keeping commitments. Your marriage will be the primary influence on their model and idea of marriage. So, your life must be marked by these principles so your kids understand how these principles get lived out as adults.
Is there a specific age to bring the sex part up with them? No, you will have to assess the age appropriateness with each child individually. Sometimes girls are ready earlier than boys (from what I have read), but it will all depend upon their emotional readiness. But at some point perhaps between 8-12 years old, there will be a time to talk about how babies are made and born into the world. If the preceding foundation principles have been adequately established, then this conversation can build upon them. Will it still be awkward? Sure, but at least it will have an anchor in the Biblical context of love, marriage, and commitment and will be consistent with all the other foundational messages you have sent to your kids. I have not had to have this part of the talk yet with either of my kids - one is 7, the other is 5. But I know it is coming, and therefore I am focusing on developing and establishing the proper principles with them.
Bottom line, your kids need to know that marriage matters to God. The world will send them conflicting signals about marriage, love, commitment, sex, and purity. So, your kids need to know that we are to be different than the world. There are lies, destruction, and hopelessness in the messages of the world about these things. God provides truth, life, and hope for them as kids and as they develop into teenagers, young adults, wives, husbands, dads, and moms.