My text field is not taking any input

I am writing a GUI application for Unity and I have text box showing which is great but for some reason I can't enter any kind of text into it.

Here is the class:

 ///  <summary>
 /// Main function where the GUI windows are created
 /// </summary>
 /// 
 void OnGUI()
 {        
     MyFirstWindow();        
 }
 
 /// <summary>
 /// Displays the background survey that the user will fill out
 /// </summary>
 void MyFirstWindow()
 {
     int windowWidth  = 800;
     int windowHeight = 600;

     //// Make a background box
     GUI.Box(new Rect((Screen.width  / 2) - (windowWidth  / 2),
                      (Screen.height / 2) - (windowHeight / 2),
                       windowWidth, windowHeight), 
                       "Hello World!");
     string  txtAge   = "";
     txtAge = GUI.TextField(new Rect(600, 355, 50, 40), txtAge, 2);
 }