Well...start as young as you can..see if they are insterested and having fun on the mat. That's most important. I started mine at 15, one at 8 and the youngest tried it at 6..but it just wasn't for him at that time. We'll try again when he get's a little older. Take your cues from the child. I think you might want to get them grappling or in some sort of jiu jitsu as well. It makes transitioning into a very physical sport like wrestling easier. They need to get used to rolling. All of mine started some sort of martial art very young, 3, 4 & 5 years old.
Well I don't know where you live but the WWF (Wisconsin Wrestling Federation) says 4 years old. but as of mom of 2 wrestlers you know your son, if he wants to and you feel he is ready go for it, but if he is on the fence hold off, or he may not want to stick with it.