John Gill Commentary 2 Chronicles 31:17

"and them that were reckoned by genealogy of the priests by their fathers` houses, and the Levites from twenty years old and upward, in their offices by their courses;" — 2 Chronicles 31:17 (ASV)

Both to the genealogy of the priests by the house of their
fathers
As to the priests registered according to their families from the age before observed, a portion was given: so to the Levites,

from twenty years old and upward, in their charges, by their courses ;
for though originally they were not admitted into the tabernacle till twenty five years of age, nor to officiate till thirty, but in David's time they were allowed at twenty years of age and upwards, (1 Chronicles 23:24) .