John Calvin Commentary Numbers 8:24

"This is that which belongeth unto the Levites: from twenty and five years old and upward they shall go in to wait upon the service in the work of the tent of meeting:" — Numbers 8:24 (ASV)

This is it, that belongeth to the Levites. The age is prescribed here when the Levites should begin and end the execution of their office. God commands them to commence in their 25th year and grants them their dismissal in their 50th. For both these provisions, there is very good reason.

If they had been admitted in early youth, their levity might have greatly detracted from the reverence due to sacred things. This is not only because those whose manhood is not yet mature are generally given to pleasure and intemperance, but also because, either by negligence, or levity, or thoughtlessness, or ignorance and error, they might have made many grievous mistakes in the service of God.

Moreover, while they were by no means fit to carry out their duties until they had attained prudence and gravity, it was also right that they should be dismissed at an appropriate time, lest they fail from old age. As we have said before, their duties were laborious and demanded bodily strength.

If, however, anyone should choose to apply this to the pastoral office, it should be generally remembered that none should be chosen for it except those who have already given proof of their moderation. Furthermore, those who diligently devote themselves to it should not be unreasonably pressured, nor should more be required of them than their ability can bear.

For some foolishly count their years, as if it were a sin to choose a pastor before his 24th year, although he might otherwise fully possess the necessary qualifications.