At what age does excercise begin to result in strength in boys and girls?

Answer 1

At birth.

I know a boy who was in an orphanage in a developing country until he was 6 months old. His neck muscles were weak probably because he hadn't had the freedom to crawl around; he was confined to a crib. Within a few weeks in an American home, his neck muscles strengthened.

Answer 2

this is the wrong answer it's not at birth