I have a problem with IComparable and the collection sort method

class Parent : IComparable<Parent>
class Child : Parent

Child a = new Child();
Child b = new Child();
a.CompareTo(b);

List<Child> l = new List<Child>();
l.Add(b);
l.Add(a);
l.Sort();

public int CompareTo(IDType other)
 {
       return this.Name.ToString().CompareTo(other.ToString());
 }

public class Parent : IComparable<Parent>
    {
        public int Age { get; set; }


        public int CompareTo(Parent other)
        {

            return this.Age.CompareTo(other.Age);
        }

    }

    public class Child : Parent
    {
        public string Name { get; set; }
    }

public class Parent : IComparable
    {
    public int Age { get; set; }


    public int CompareTo(object other)
    {
        Parent p = other as Parent;
        return this.Age.CompareTo(p.Age);
    }

}

    public class Child : Parent
    {
        public string Name { get; set; }
    }

List<Child> c = new List<Child>();
    c.Add(new Child { Age = 10, Name = "Alex" });
    c.Add(new Child { Age = 6, Name = "Jack" });
    c.Add(new Child { Age = 15, Name = "Bob" });

    c.Sort();