Resistance training as a kid/adolescent has been proven to increase your bone density peak.
What does it mean?
It means when you are a child and then teenager your bones develop and grow and with doing some good quality (form form form) resistance training you can increase density of your bones and have them stronger for the rest of your life.
It is true that bone keeps changing even when we done with crazy teenager years, but it will never be so active and as we get older bone metabolism will start to go in a reverse direction.
So don't be blinded by misunderstanding, not understanding, misconception,
evidence is there: children can and could do resistance training!