What does the verse teach about the Yichus of the Kohanim and Leviyim?
'Rashi': Together with the coming verse, it teaches that they gave enough Terumos and Ma'aseros for the wife, sons and daughters of the Kohen or Levi. This applies to those who came to serve, and those in their cities.
Why does it specify 20 years old?
Redak: [Chizkiyah] summonsed Leviyim to come before the age to serve 1 , so they will be used to Avodas Hashem, for all forgot it when the Avodah ceased in the days of the evil kings.
Malbim: For Kohanim and Leviyim, this is the age to receive for himself. If he had sons above three, he received also for them.
The Torah said that Leviyim begin learning Avodah at 25, and begin to serve at 30! Perhaps Chizkiyah allowed more time to learn, for the teachers were not used to the Avodah!
What do we learn from "b'Mishmerosam b'Machlekoseihem"?
Malbim: Each received according to what his Mishmar merited, e.g. meat and skins of Kodshim.
