Unlike the rest of the Jewish men, who were counted from age 20, the Levites were counted from the age of one month. G‑d then told Moses to count all the firstborn male non-Levites who were at least one month old. The Levites were replacing the firstborn as employees of the Tabernacle since the firstborn had forfeited this privilege by participating in the incident of the Golden Calf.
Know Him in All Your Ways
קַח אֶת הַלְוִיִּם תַּחַת כָּל בְּכוֹר בִּבְנֵי יִשְׂרָאֵל וגו': (במדבר ג:מה)
[G‑d told Moses,] “Take the Levites in place of all the Israelite firstborn.” Numbers 3:45

The other tribes were counted only from age twenty – the age at which they were old enough to serve in the army. The Levites’ task, however, was to protect the Tabernacle, and any increase in their population – even of newborn infants – helped them in this task. Therefore, they were counted virtually from birth.

We can all join the legions of the Levites in a spiritual sense, by nurturing our intrinsic connection to G‑d. This connection is not affected by any variations in time, age, or environment. It therefore enables us to “know Him in all your ways,” down to the simplest aspects of life that a mature adult shares with the smallest child.1