Numbers 1:3

3. Twenty years From the son of twenty years. This was the law ever afterwards. The word able does not occur in the Hebrew. If it had been written, the names of the sick and aged men would not have been enrolled, and the enumerators must have added to their functions those of surgical examiners. Exemptions from military service were established on Divine authority several years afterwards. See Deuteronomy 20:5-8. There is not the least hint in the text of an upper limit to the military age. Yet Josephus asserts that this census enrolled only the fighting men between twenty and fifty years of age. It is quite probable that subsequent legislation fixed the maximum age at fifty years, and that Josephus ( Ant., Numbers 3:12 ; Numbers 3:4) has carried the law back too far. For if there had been such a maximum age it would have been expressed with the minimum. Moreover, since the grand total of this military muster-roll is exactly the same as that of the poll-offering in Exodus 38:26, it would follow, according to Josephus’s statement, that all above fifty years must have been exempted from the offering. Of this immunity there is no hint, either in the original command (Exodus 30:11-16) or in the report of its execution. Exodus 38:26.

Thou and Aaron shall number Number is not the idea of the Hebrew, but rather review, muster, arrange. Moses and Aaron were to constitute a board of superintendence, while the details of the census were to be attended to by a commission of twelve distinguished men, one from each tribe. These would be sufficient, for the chief labour of the census had already been performed. In arranging for the service of the newly-erected tabernacle, nine months before, it had been ordained that each person enumerated should pay half a shekel, about twenty-seven cents, atonement money, “a ransom for his soul unto the Lord.” See Exodus 30:12. Since the atonement money had already been offered, (Exodus 38:25-28,) we infer that a census had already been taken in the tribes, or in subdivisions of tribes, and records made of the results. The office of the census board, just appointed by God, was, probably, to collect and classify those registrations which had already been made, and draw out the people into military divisions.