Matthew Poole and continuators on 1 Chronicles 23:3

3 Now the Levites were numbred from the age of thirty years and upward : and their numbers by their polls, man by man, was thirty and eight thousand.

Not onely till fifty, as it was appointed, Num. 4. 2, 3. but even till their Death: For that was but a Temporary Law grounded upon a special Reason, because the Levites were employed in carrying the Tabernacle and Sacred Vessels from place to place; and therefore God would have them freed from those Burdens when they came to feel the Infirmities of Age: Which Reason wholly ceasing upon the Building of the Temple, and their Work being far easier than it had been, and their Service being more a Priviledge than a Burden, their time of Service is justly and fitly prolonged.

Marginal references: Numb. 4. 3.