They waste their potential, their abilities, their health, their future. In some cases, they waste their lives. For the sake of our discussion, I consider a child who has rejected the values of his family to be a prodigal, even if that child still lives at home, even if that child is 12 years old. I believe rebellion comes from a combination of personality, environment, genetics and the basic sinfulness we all have. But in reality, every child is different.
Every child is born into a different family.
The family dynamics change as more children come into the family. Parents react differently to their different children. Maybe you move or change jobs or someone gets sick. All of those changes affect the atmosphere of the family. There are also factors like mental illness and addiction that play into this.