I personally remember learning about WW2 and the Holocaust in grade 6, when I read Number the Stars. I think I had heard of it earlier, but didn't know much about the topic. I think it can be introduced in general terms to younger children. I also think you need to take the environment where you live in to consideration - if you live in an area with a large Jewish population or a large European immigrant population it is possible your child will be going to school with people who learned about the Holocaust at a younger age or through family stories passed down from grandparents or great-grandparents who were survivors themselves. In that case, it might be appropriate to discuss the Holocaust with your child at a younger age in case it is brought up in discussion.