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.