Charles Ellicott Commentary Exodus 30:14

"Every one that passeth over unto them that are numbered, from twenty years old and upward, shall give the offering of Jehovah." — Exodus 30:14 (ASV)

From twenty years old and above. — A Hebrew was not reckoned full grown till twenty. At twenty the liability to military service began (Numbers 1:3; 2 Chronicles 25:5).

At twenty the Levites commenced their service in the sanctuary (1 Chronicles 23:24–27; 2 Chronicles 31:17; Ezra 3:8).