At what age the kid should understand the value of money

Hi Friends

Its a question that came to my mind when i became a mom....(now my kid is 4).May be its too early to think, still i will like to ask u all...
What is the right age of a kid to make him understand about the value of money??
The main aim is, in Today's world,we parents r very busy and incoming in good figures...and most important thing is we dont have enough time to spend with our kids.So whenever they r demanding anything...we r just fulfilling them....But sometimes i think they should also know the value of money, otherwise the expectation will grow with their age...and i dont want to happen this with my kid.
To be frank, i have seen some kids in my family are so demanding...at the age of 9/10 they r demanding for expensive things.I know the parents can afford and they feel if they will not buy those things, their kids r feeling bad in front of their friends.I am scared of this situation.We r OK to fulfill everything that my kid will want...but i dont want him to be demanding.I want him to explain that money doesnt come easily...so that he shd have a respect for money in his mind n im sure it will definitely help him to make a good future.
I dont know whatever i am thinking is right or not....but i want some views.Also how you all r guiding ur kids and at what age..pls share with me.