When I began writing books about puberty for kids, I wrote my first book for young teens. I didn’t publish it because I realized that I should start at a younger age.
After much research and testing with kids and parents, I concluded that 5-7 years old is the most receptive age to teach them about their reproductive structures. Young girls are usually not embarrassed as you talk about ovaries and a uterus. Little boys are fascinated by the response of the testicles to different temperatures and many have noticed the changes in themselves.
Recently, I was reminded of my enthusiasm for this age group when I taught two different kindergarten classes about their bones. Using plastic models to demonstrate their anatomy, the kids and I had a great time!