John Wesley on 1 Chronicles 23:24

Twenty years - As the Levites were anciently numbered from two several times, from the twenty fifth year of their age, and from the thirtieth, Numb 4:3 8:24. In like manner they are here numbered both from their twentieth year, when they were solemnly prepared for, and instructed, and by degrees exercised in some parts of their work; and from their thirtieth year, when they were admitted to the full exercise of their office. And the reason why they were now sooner admitted to service than they had been formerly, is given in the next verses because now their work was more easy, being wholly discharged from that burdensome work of carrying the tabernacle. Besides the people of Israel were multiplied: therefore more hands were necessary, that every Israelite who brought an offering, might find a Levite ready to assist him.