There's no doubt that some supplement companies have made headlines for the wrong reasons in recent years. And pretty much every lifter can tell you a story about the time they took a stim-heavy pre-workout that was a bit more intense than they expected. However, as long as you follow a few straightforward rules, you can have a safe and productive training session after taking a pre-workout.
First and foremost, go in with your eyes open! Know what you're taking. Make sure you buy your pre-workout from a reputable, established company, preferably one whose products have been tested and approved by an independent third party. Informed-Choice is one such testing organization. Informed-Choice tests about 18,000 samples a year for contamination by banned substances, and to verify that what is on the label of your pre-workout matches exactly what's in the bottle.
If you know what you're taking, the next step is to know yourself, and why you're taking a pre-workout in the first place. Is it to get a specific extra edge for your specific workouts, or is it just a way to try to motivate yourself into training? Are you comfortable with stimulants, or do you know they make you feel anxious and unpleasant, or struggle with insomnia? Is your training and nutrition in order, or are you hoping that a pre-workout will magically give you results? Answer these questions before you click "buy," and remember that a pre-workout is just the icing on the cake. Training consistently and eating right—those are the real keys.
More specifically to young athletes, there are no legal age limits for the use of pre-workouts nor for any other exercise-related supplement. And sure, a pre-workout may be "safe" for, say, a teen athlete, but that doesn't mean it's necessary or even helpful. A teen athlete or lifter would get more from simply eating right and training regularly than from taking any particular supplement—particularly one that can cause rapid heart rate and other side effects that could hinder an otherwise potentially productive workout.
And no matter what your age, if you have problems with your heart, be sure to consult a physician before taking a pre-workout or any other supplement.