John Calvin Commentary Numbers 4:1

"And Jehovah spake unto Moses and unto Aaron, saying," — Numbers 4:1 (ASV)

And the Lord spoke to Moses and to Aaron. This census had a different objective from the former one we have just been considering, namely, that an equal distribution of the responsibilities should be made in proportion to the number of individuals.

First, regarding age, a distinction must be observed between this tribe and the others; for we have already seen that all the Israelites over twenty years of age were numbered because they were then able to bear arms. But it was not without reason that a more mature age was required for the Levites, so that they should not begin to discharge their ministry before their thirtieth year.

For not only are bodily strength and vigor required for spiritual warfare, but seriousness and gravity also. If they had been admitted in their youth, their frivolity might have diminished the reverence due to sacred things, since the young are often led to act intemperately by their fervor and licentiousness. Therefore, access to the sanctuary was not permitted them until they had matured into men; for by their thirtieth year, men ought to have become so composed that it is shameful and inexcusable for them to give way to the recklessness of youth.

From their fiftieth year, they were released from their duties, since sloth and inactivity generally accompany old age. The case is different concerning 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 lose respect on account of the idleness and drowsiness of its ministers.