FIRST, check with your pediatrician to make sure it’s safe to exercise. If he/she clears you to workout and you are under 14, then I recommend working out using body weight exercises like pushups and pullups, sit ups, and running until you are a little older. Once you are 14, you can add in weights, resistance bands and other types of training. As long as you eat to maintain a healthy weight, your growth will NOT be stunted. Do not take supplements outside of medicines a doctor prescribed to you personally and a multivitamin. See if your gym or fitness center offers an orientation workout to show you how to get started. Most gyms offer this free to new customers as a one time gift. Some facilities even offer fitness programs for teens, so check into that option as well. Good Luck!
Fortunately, that is not true.
Engaging in physical activity is something that we do from the time that we are children.
Can you imagine what would happen if a child never ran in the park, played on the monkey bars, jumped rope, rode a bike, swam, skiied, skipped, tumbled, played baseball, football, hockey and all the wonderful sports we are introduced to when we are kids? We would grow properly.
Engaging in physical activity, whether it be in a gym or outdoors is one of the best things an individual can to do prevent disease and grow to be a fit, strong individual.
I hope this answers your question.
Physical activity (regardless of its form) should not stunt growth in anyway. What should be more of a concern is getting kids to specialize in specific sports way too early in their development (as opposed to having kids develop general fitness and gross motor movements). Early specialization leads to imbalanced physical development and increases the risk of injuries.
Should a child be exercising in a gym, they need to make sure that they receive a general fitness program that covers all aspect of physical fitness, or at least engage in a variety of activities to ensure that all aspect of physical fitness are being met.