Numbers 4:1

1.And the Lord spake unto Moses and unto Aaron This census had a different object from the former one, which we have just been considering, viz., that an equal distribution of the charges should be made in proportion to the number of the individuals. First, as regarded age, a distinction must be observed between this tribe and the others; for we have already seen that all the Israelites above twenty years of age were numbered, because they were then fit to bear arms. But it was not without reason that a more mature age was required in the case of the Levites, so that they should not begin to discharge their ministry before their thirtieth year. For not only is strength and rigor of body requisite for spiritual warfare, but seriousness and gravity also. If they had been admitted in their youth, their levity might have detracted from the reverence due to sacred things, since the young are often led to act intemperately by their fervor and licentiousness. Access to the sanctuary, therefore, was not permitted them till they had grown up to be men; for by their thirtieth year men ought to have become so staid, as that it shall be base and inexcusable for them to give way to the wantonness of youth.

From their fiftieth year they were released from their duties; since sloth and inactivity generally accompany old age. The case is different as to war, because we find many who are vigorous after their fiftieth year. Furthermore, since religion is more precious than all earthly affairs, diligent care was to be taken lest it should fall into disesteem on account of the idleness and somnolency of its ministers.