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

Why were the Leviim counted from one month and up? Versus the rest of the Yidden, who were counted from age twenty and up? The Yidden are counted for their personal יחוס – status, which can only be created from age twenty when a person is considered an independent adult.  Whereas a Levi receives his status based on his lineage, the Leviim are considered a מיוחס – special, from one month old.