12 YEAR OLD. A 12 year old is sometimes old enough to help out when mom just have a baby. She has to be taught on how to do things and I am sure she will learn fast.
Cultural issue. In my travels I have seen cultures that routinely gave 8-9 yo such responsibilities, but they would be classified as the third world. While a 12yo can be helpful, he or she does not have the mental maturity to understand the long term impact of choices seen in the 15 and older crowd. Some may be more mature earlier than others, but I don't think it is wise to burden a 12 yo with that responsibility.