כל זכר מבן חדש ומעלה תפקדם (במדבר ג:טו)

The Leviyim (Levites) were counted from the young age of thirty days, whereas the Yidden were counted only from age twenty; why was this so?  Says the Imrei Chaim of Viznitz: The Yidden could only be counted from the age of twenty when they had reached full adulthood – when one’s pedigree as an independent, responsible, and upright person is established.  The Leviyim, however, inherited their status as a Levite from birth – their Yichus (lineage) preceding them – and so could already be counted from when they were one month of age.