Numbers 4:43

From thirty years old and upward even unto fifty years old, every one that entereth into the service, for the work in the tabernacle of the congregation,

Cross-reference

Numbers 4:35 gives the same age range (30-50) for Kohathites. Identical phrase for a different Levite family.

1 Chronicles 23:3 also lists Levites from age 30 upward for service, matching the age threshold for Merarites here.