In 1 Chronicles 23:24, the Levites are numbered from twenty years old; this verse gives the reason — by David's last words.

2 Samuel 23:1 introduces 'the last words of David' — the phrase cited here as authority for the age change.

Numbers 4:35 sets Levite service from thirty years old — contrasting the twenty-year minimum established here.

Numbers 4:47 also specifies thirty as the starting age — directly contrasting the twenty-year minimum in this verse.

2 Chronicles 31:17 records Levites from twenty years old in Hezekiah's reign — later application of this age change.