There are three major variables to consider any time you invest money: your natural ability to deal with risk, the amount of risk you can afford to take, and the amount of time until you will need your money. All three are important considerations.
I see the teen years as the prime time for several reasons. First, although some teens wean themselves from their parents’ finances faster than others, most have some money of their own, whether that’s from a part-time job, allowance, or gifts. And there’s nothing like having skin in the game to make their education more meaningful.