* from a month old.
   The males of all the other tribes were numbered from twenty
   years old and upwards; but, had the Levites been numbered in
   this way, they would not have been nearly equal in number to
   the first-born of the twelve tribes.  Add to this, that as
   there must have been first-born of all ages in the other
   tribes, it was necessary that the Levites, who were to be
   their substitutes, should also be of all ages; and it appears
   to have been partly on this ground, that the Levites were
   numbered from a month old and upwards.

  4:38-40