What is a Good Age for a Child to begin getting an Allowance?

My son is learning about money and is real ecstatic everytime I let him keep some change or dollars. However I do not want him to start becoming obsessed with money. I do know it makes him feel good to have some money to buy something. I want him to earn it as well, what is a good age to start giving an allowabce, he is 6 years. Thanks