Numbers 8

“This is what pertains to the Levites: From twenty-five years old and above one may enter to perform service in the work of the tabernacle of meeting;  and at the age of fifty years they must cease performing this work, and shall work no more. They may minister with their brethren in the tabernacle of meeting, to attend to needs, but they themselves shall do no work. Thus you shall do to the Levites regarding their duties.” vv.24-26

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If you remember from chapter four of Numbers the age of the duties of the Levites were from 30-50 years old, so why here does it say starting at 25 years old?  It is supposed that five years were anticipated for preparation for service of in the Tabernacle.  We do see that after age 50 they could minister to their brothers in the tabernacle, just do none of the actual work duties.  Maybe the 50+ year olds trained the 25-30 year olds?  We do see David change the age to 20 years old in 1 Chronicles 23,

 "These were the sons of Levi by their fathers’ houses—the heads of the fathers’ houses as they were counted individually by the number of their names, who did the work for the service of the house of the Lord, from the age of twenty years and above. For David said, “The Lord God of Israel has given rest to His people, that they may dwell in Jerusalem forever”;  and also to the Levites, “They shall no longer carry the tabernacle, or any of the articles for its service.”  For by the last words of David the Levites were numbered from twenty years old and above."  

So later there had been a change in age since the tabernacle was no longer being moved through the desert but remain in the City of David.
Early retirement is what we might say today seeing that at age 50 they were to no longer work in the service of the tabernacle.  How merciful and compassionate is our God!  He who knows the heavy labor men are able to do in their prime but does not require that same endurance as they advance in years and slow in agility.  I'm not saying 50 years old is  when everything is down hill for I know many very fit and able 60 and 70 year olds, but as our bodies are continually decaying we need to care for our bodies to have a long lasting life, especially in advanced years.  What age bracket are you in right now?  How are you training, working or ministering for the Lord to the best of your ability?