Concurrency in Ada

[Stribor40 <ikamzic@gmail.com>]

I have this program from online tutorial that says that tasks will run as soon as program starts. My question is when does this program starts on what line if code
WITH Ada.Text_IO;                  --  Include Text_IO Library
WITH Ada.Integer_Text_IO;          --  Include Integer Text_IO Library
PROCEDURE task_demo_01 IS
                                   --  Task Type Specification
     TASK TYPE intro_task (message : Integer);

     TASK BODY intro_task IS       --  Task Body Definition
     BEGIN
          FOR count IN 1..5 LOOP
               Ada.Text_IO.put (Item => "Display from Task ");
               Ada.Integer_Text_IO.put (Item => message, Width => 1);
               Ada.Text_IO.new_line;
          END LOOP;
     END intro_task;
     --  Unlike procedures, these tasks are not called.
     --  These three tasks are activated once the program begins.
     Task_1 : intro_task (message => 1);
     Task_2 : intro_task (message => 2);
     Task_3 : intro_task (message => 3);
BEGIN                                           
     NULL;                                   
END task_demo_01;

Do tasks run all at same tine or they are scheduled by OS which would mean anytime you run this program output would be different?