The Alshich on Ki Tetze teaches that until the age of 13 a person has only an evil inclination. The good inclination comes only at Bar Mitzva. Hence, the yetzer hara is likened to an old seasoned warrior (who has total control of the body before the yetzer tov comes), while the yetzer tov is likened to a young lad. (See there.)
The Alshich continues to say that for the yetzer tov to conquer the yetzer hara is a formidable task, and is the "tachlis hagevura" (the ultimate mightiness).
Why did God make a system like this? For free will to be balanced it seems logical that both should exist simultaneously and neither should have any advantage over the other.