Serve the Lord from 20 years old

1 Chronicles 23:14-32
Moses is son of Amram, Amram is son of Kohath.
The descendants of Levi, the workers twenty years old or more who served in the temple of the Lord.
The Levites no longer need to carry the tabernacle or any of the articles used in its service. According to the last instructions of David. The duty of the Levites was to help Aaron's descendants in their service of the temples, the side rooms, the purification of all sacred things and the performance of other duties at the house of God. And so the Levites carried out their responsibilities for the Tent of Meeting, for the Holy Place and under their brothers the descendants of Aaron, for the service of the temple of the Lord.
From the earliest days of desert-wandering, the job of the Levites had been the care and transport of the Tabernacle. They had also served the priests - in later times at the many shrines scattered throughout the land. Now the Ark was to have a permanent home, and worship was to be centralized in the Temple at Jerusalem. So David allocates new duties to the Levites: the care-taking and maintenance of the Temple; appointments as magistrates, janitors, musicians and choristers; and general assistance of the priests.
Thirty was the age at which a Levite began his work, David changed this to 20, once the Temple was completed.
Why?
David was walking with the Lord and be loved on in present of the Lord, so he set last service instruction of Levites.
How?
Serve the Lord may more younger from 20 years old.