Numbers 4:1-3 NASB
Then the Lord spoke to Moses and to Aaron, saying, "Take a census of the descendants of Kohath from among the sons of Levi, by their families, by their fathers' households, from thirty years and upward, even to fifty years old, all who enter the service to do the work in the tent of meeting.
Numbers 8:23-25 NASB
Now the Lord spoke to Moses, saying, "This is what applies to the Levites: from twenty-five years old and upward they shall enter to perform service in the work of the tent of meeting. But at the age of fifty years they shall retire from service in the work and not work any more.
When the census was taken of the Levite's the age restriction was thirty to fifty years who came to serve in the work of the tent meeting,but when they were consecrated & purified the age restriction seems to have changed to twenty five years & over.
How can we reconcile the supposed contradiction?