A study by Cambridge University for the Money Advice Service into 'Habit formation and learning in young children' shows that by age of seven most children have grasped the concept of money and how to count it. They will also come to have an understanding that money can be exchanged for goods. There are many different techniques that can influence children to help them handle money that will help them later on in life.
The guide is available here and there are no hard and fast rules that should be followed but it's an excellent resource for parents.
It aims to provide an insight and ideas into about how to prepare the next generation to mange their money.