fitness in young teenagers

Much has been talked about fitness in adolescents, there are two positions on this issue, on the one hand there are people who advocate healthy living and good nutrition in young people, but there are others who say that exercise at an early age has repercussions on health problems in the future, such as low weight, and hormonal imbalances because when entering the fitness world you get to consume substances that alter the metabolism of the person. I am a fitness guy for 5 years I practice this lifestyle and My opinion about this is that it will always be beneficial for the body to exercise and to take a good diet as long as the excesses are not reached, so I would like to ask you which side you are on, for or against.