It has to do with genetics. When your bones grow, then the information of how much they grew is stored in your DNA. When a bay is born, a small percentage of the of the parents' DNA is transferred into the baby. Included is a small percentage of the bone growth information from your parents. If its a boy, its likely going to be of a similar height to the father, in addition to the total growth of the child. A similar thing happens to girls.
This is how we've grown by miniscule amounts over thousands of years. Eventually, 9 feet will be the average height. And think of the small percentage of parents' DNA of being like a "minimum height requierment" for bones. It tells your body about how tall it needs to be. Theres math involved, but I'm not going into details.