1 Chronicles 23:27

For by the last words of David the Levites were numbered from twenty years old and above:

The chapters of 1 Chronicles 21-23 belong to the prosperous years of David's reign. By David's last direction after the temple was built, the Levites were numbered for service from twenty years old and above. They began the service of the sanctuary at twenty.

At fifty they returned from the warfare of the service, meaning they were not discharged but still had charge of the work while released from arduous duties. Their work was waiting on the priests in the sanctuary service, preparing the showbread, often leading the singing, collecting the tithe, and doing a large amount of work connected with the service of the Lord. That work was largely confined to the court. It typified the work of the present-day gospel ministry.