Rashi explains that the minimum age for serving in the army was twenty-years-old. We may wonder why it was necessary to wait so many years past the age of bar mitzvah to enter the Jewish army. Even as teenagers the youths were probably physically mature and competent to fight as soldiers. Horav Tzvi Hirsch Farber, z.l., offers a homiletic response to this question. A young man, a rgb, is also called an okg. This term is derived from the fact that he is still an enigma, his actions are ohnkgb, concealed. Since he has not quite matured, we are still in doubt concerning the underlying motives of his actions.
Bnei Yisrael did not win the war on the battlefield against the pagan armies as a result of the physical prowess of its soldiers. The outcome of the war was determined by the spiritual ascendency of its soldiers, their religious conviction and mitzvah observance. Bnei Yisrael experienced miraculous victories. Only a very special soldier would merit such successful deliverance. When a soldier had reached the age of twenty years old, he was no longer an okg. His actions and convictions were no longer cloaked in obscurity. He had the qualifications to be the vehicle for Bnei Yisrael’s successful campaigns.