Did the Levites begin at thirty, at twenty-five, or at twenty?

The Bible gives all three, and this chapter gives two of them. "The Levites were counted from thirty years old and upward" (23:3). Then the list ends with those who did the work "from twenty years old and upward" (23:24), and the chapter says, "For by the last words of David the sons of Levi were counted, from twenty years old and upward" (23:27). Numbers counted them "from thirty years old and upward even until fifty years old" (Numbers 4:3), and in Numbers 8 the LORD says, "from twenty-five years old and upward they shall go in to wait on the service in the work of the Tent of Meeting" (Numbers 8:24). Later books have twenty again: "the Levites from twenty years old and upward" (2 Chronicles 31:17), and those who had come out of the captivity "appointed the Levites, from twenty years old and upward, to have the oversight of the work of the LORD’s house" (Ezra 3:8). This chapter puts the count from twenty right after David's words, "Also the Levites will no longer need to carry the tabernacle and all its vessels for its service" (23:26), and says no more about it. Numbers 4's honest question sets 4:3 beside 8:24 too. Neither book says how the ages fit together, and this answer doesn't either.