As a teacher, parent, former youth pastor, and teen/family counselor, I have often been asked the when question regarding teaching something to children. Examples: When should we talk about sex? When should I tell them about Santa Claus? When will they understand this or that about God? When should I teach them about faith, serving others, the Holy Spirit, etc.? (We have many of the same discussions in the education realm - When are they ready to write in cursive? Or, should we at all? When should we introduce fractions?)
There is a lot about the brain and human development that factors in here, and while that is very important, we can't all be neurologists or developmental psychologists. Besides, even within those fields among the experts, there is so much debate and disagreement. So, I have only one suggestion here - a "rule of thumb" if you will - to help us. The idea actually comes from the teachings of Scripture thousands of years ago.
In the Old Testament, we come across a phrase more than once that reads like this: "And when your children ask you...then tell them...". Someone asked me recently when teaching takes place with children in the organic church life. My answer - "When you breathe." The spiritual formation and overall education of our children is a constant, daily activity. Don't be fooled into thinking that learning takes place "at school" or that worship takes place "at church." That mindset will rob you of the rich experience of discipling your children, and frankly, has probably done more damage than good in the name of religion.
Instead, we can be looking and listening for the teachable moments, what Dr. James Dobson referred to as "incidental learning," the type of learning that takes place in the midst of everyday life. Of course, this means we have to be present with our children, not just physically but also emotionally and mentally. (This is an area I have to constantly work on.)
In sum, a good guiding point as to when children are ready to learn about something is whenever they ask about it. And the assumption in the Old Testament seems to be that if you're really living faithfully for God, then that alone will prompt them to ask questions. There is no need to wait for them to be put in artificial settings (classrooms, Sunday School, youth groups, etc. - although these can sometimes help) for them to receive instruction. And the icing on the cake is that while we are looking for these ongoing opportunities to pass on truth, faith and love to the next generation, we just might learn something from them, as well.