As soon as possible! The key idea is not just to teach them about saving money. It should be more about teaching them to invest, accept delayed gratification and seeing the big picture. I know it may be far-fetched to be teaching all these to a kid. But I think we should teach them whenever there is an opportunity.
I do not have kid of my own, however, I recalled there was a time when my niece wanted to get something from a toy store. She was 7 years old back then. At that time, I asked her why did she want the toy. Being a kid, she said something like, "I just want it". I took the opportunity to teach her that there must be a good reason for everything you need to spend money on. Because her parents' money doesn't come easy. Every dollar spent on unnecessary things, could mean losing a dollar on the necessities. I am not sure if she got it at that time, but at least she didn't insist on getting the toy that day.
From the example I described, you can see that teaching kids such concepts earlier will also benefit yourself. That is why it is so important to start ASAP and find every chance to impart these knowledge to them. Looking back, I wished my parents could have taught me the concept of investing and saving earlier in my childhood. I will probably be in a much better financial position than I am now if I had started earlier.