I found it to be the same answer to "when do you potty train your child?" with the answer being, "when the child is ready." We have two kids, and the oldest is eight. He has been learning about money for about 2 years and it is starting to sink in. I am not super sure if he would be in the same place if we just started this year. He is ready to learn and he understands math better. Our 3 year old does not spend money, but we started her too to be fair. She is more ready to handle finances than her older brother, honestly.
We give them $1 per year of age per week. So the oldest just got a raise to $32 a month. We also believe he needs to learn how to use his money, so he has to put money in the bank ($3) to save and give money to the church ($3). These are the general philosophies we have, so we pass them on to him.