Children are never too young to learn, I was fortunate enough to have a hand illustrated book for children from about the age of four years old that they could look at and ask questions.
Questions do change as they grow older and I was still having the odd few come through in their late teens, so no as soon as they ask, be honest with answers taking the age into account @therneau
Yup, indeed!! The important thing is that he knows it's ok to talk to daddy instead of getting heaven knows what advise from his peers :)