Matthew Henry on Numbers 4:1-3

The middle-aged men of the tribe of Levi, all from thirty years old to fifty, were to be employed in the service of the tabernacle. The service of God requires the best of our strength, and the prime portion of our time, which cannot be better spent than to the honour of Him who is the First and Best. And the service of God should be done when we are most lively and active. Those do not consider this who put off repentance to old age, and so leave the best work to be done in the worst time.