C# 2d array's System.NullReferenceException: Object reference not set to an instance of an object

Question

I have written C# codes that make use of 2d arrays. Here are the codes:

class HelloWorld
{
  static void Main(string[] args)
  {

      Person[,] _personArray = new Person[3,3];
     int ages =  _personArray[1,1].age; 
           Console.WriteLine(ages.ToString());
     Console.ReadKey();
  }
}

public class Person
{
   public int age = 21;
}

However, it gives me an exception that says:

"Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object at HelloWorldApplication.HelloWorld.Main (System.String[] args) [0x00000] in :0

[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object at HelloWorldApplication.HelloWorld.Main (System.String[] args) [0x00000] in :0 "

Can anyone please help me and tell me what is wrong with my codes? Your help will be appreciated :)