John Wesley on 1 Chronicles 23:3

Upwards - Not only 'till fifty, as it was appointed, Numb 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, their work being far easier than it had been, and their service being more a privilege than a burden, their time of service is justly prolonged.