The tribes of Levi are counted twice: (a) from age one month with no upper age limit, and (b) from age 30 to 50. Why is the first count needed? (This described in parshot Bamidbar and Naso.)
1 Answer
The first census allowed the redemption of the first-borns against the Leviim. Each of them were counted from age one month and up.
One month is the standard age in Jewish law for verifying that a baby is viable; that may explain this feature for both groups (Rashbam).