All that are able to go forth to war— Hence it appears, that the aged and infirm, such as were unable to go to war, were not numbered: and, accordingly, Josephus tells us, that after fifty years they were not obliged to pay the half shekel which was due upon such musters. Among several other nations, as well as the Jews, particularly the Romans, all who were of age to bear arms were obliged, upon some occasions, to go forth to battle. Hence it is, that we read of the kings of Israel bringing such numerous armies into the field.
Related Commentaries of Numbers 1:3
Numbers 1:3
3 From twenty years old and upward, all that are able to go forth to war in Israel: thou and Aaron shall number them by their armies.