We have had a reasonably detailed sex talk with DS who was probably 7 at the time, because he knew that people did something to make babies, he knew it involved a man and a woman, but he wanted to know more about the mechanics of it (so to speak!). To be honest, once I had explained it he said "OK, that's what <insert friend's name> said as well". So I think by avoiding it you let them be informed by other people. I was impersonal with DS and tried to explain it from a simple biological viewpoint. E.g. the man has a seed, the woman has an egg. The penis goes into the vagina, which is how the seed reaches and fertilises the egg, and that grows to become a baby. I also explained that the process is hard to understand because it only works when the body is turning into an adult because the body works differently then. He seemed satisfied and hasn't really asked anything more.

Recently, DS was using the word gender to describe whether an animal is a boy or girl, but in animals the word sex is used and gender is for people (because it has physical, cultural and psychological elements), so it really is just a word. I use it all the time with DS (because I work with animals and he is interested in my work) and he understands it has two different meanings. I don't personally think 6 is too young to understand the basics. The whole emotional / pleasurable side of sex is something entirely different and appropriate for an older age (in my opinion)