class Person
{
    private int age = 0;
    private int defaultAge = 25;

    public Person()
    {
        this.age = CalculateAge(this.defaultAge); // Possible
        CalculateAge(); // Impossible
    }

    public static int CalculateAge(int age)
    {
        return age;
    }

    public static void CalculateAge()
    {
        // Cannot reference instance fields directly
        age = defaultAge;
    }
}