You couldn't just join the army and go right into the battle field, you have to go through major training and tests. If you were sent right out into battle you would be killed in the front lines, or you might give up and change sides!
This is what people are doing with their children today. They send them right out into battle when they say that they have to learn to get along with the world. If our parents don't train us to be strong in our beliefs before they send us out into "battle" we would die or change sides!
I am so thankful to God for giving me parents who know how to train us up in the Lord!