Java - How To Make Static Variable Name To Be Dynamic

Suppose I have a class child

public class child{
public static int age = 1;
}

And I am using class child static variable age in class school

public class school{
int var_age;
public school(){ //school constructor
var_age = child.age;
}
}

Value in age of child class could be any of these below depending on some logic:

public static int age = 1;
public static int age = 2;

How could i achieve this where should i apply that logic? Also it is mandatory for class school code to remain same.