In my opinion, it is not a bad idea to start even before the teenage years. But whether you start before or during the teen years, the point is the same -- if people learn smart financial habits when they are young, they are more likely to continue to practice those habits when they get older and their financial decisions are more important.
I think that our economy would be better off if more people had learned good financial habits when they were young. A lot of our trouble today has happened because people got too far in debt because they bought more than they could really afford.
The reason I mention this is that it shows why good financial habits are important. Bad financial decisions clearly hurt individuals, but they can also hurt the country as a whole. Therefore, it is important both to individuals and to the whole society that teens learn smart financial habits.
In general, earlier in life a habit is formed, it is easier to form and more difficult to loose. Also, once a bad habit is formed, it difficult to change it it and replace with good habits. Therefore, it is better to start develop all good habits as early as practical. Smart or good financial habits is no exception to this general rule. Developing such habits early in life enables an individual to develop financial discipline and skill that helps them throughout their lives.
Good financial habits mean that a person uses whatever money (financial resources) he or she has is allocated wisely to different purposes that taking in consideration the immediate requirements as well as long term benefits that can be obtained. Frequently, this becomes difficult when a person has acquired the habit of spending impulsively at the first opportunity of spending, without considering the alternative, better uses that may present themselves later. Good financial discipline also means not spending beyond your means. This kind of discipline is also heavily dependent on habits formed early. Therefore, all persons, including teenagers, will do well to develop smart financial habits as early in their lives as possible. I see no harm in parents teaching their children to spend wisely and to save for the future, soon after teaching them to buy things independently.
Bad habits die hard. It is essential for teenagers to learn good habits before they become adult and be hit with a reality that not every dollar you have can be spent on clothes or games. It is best to learn restraint while they are still growing because that trait will follow them into adulthood and they will not have such a difficult time adjusting to adult hood and financial responsibility.