The Torah asks Moshe to count the tribe of Levi, starting with boys aged 30 days and older. Why would the minimum age for the Levi census be different from those of the regular census (men aged 20 and older)?
Rav Moshe Feinstein suggests that while most of the populace was too busy working to educate their children, it was not until the age of 20 that one could determine if the children would reach the spiritual level required to represent the nation. On the other hand, the tribe of Levi did not have to work the fields and were, therefore, able to devote all their time to raising their children, imbuing them with love and dedication to serving the nation from an early age.
We are lucky to be living in times where we are able to spend quality time with our children and families, imparting our values and ensuring that every one of us is counted, and counts.