Numbers 8:25

Modern

But at the age of fifty, they must retire from performing the work and no longer serve.

Traditional

And from the age of fifty years they shall cease waiting upon the service thereof, and shall serve no more:

Literal

And from a son of fifty years, they must cease from the host of the service, and shall not serve anymore.

Latin

Cumque quinquagesimum annum ætatis impleverint, servire cessabunt,

Hebrew

English

But at the age of fifty, they must retire from performing the work and no longer serve.

These are guidelines for the length of the Levites' service.
Whereas 4:3 says their service in the Tent of Meeting started at age 30, 8:24 says age 25. This difference might show changes in age requirements over time, depending on how many Levite men were available. Those aged 25–29 might have been unofficial workers or apprentices.
Both passages set the retirement age at 50. Retired Levites could serve as guards of the Tent of Meeting (see Psalm 84:10).

Numbers 4:23
…from thirty to fifty years old, counting everyone who comes to serve in the work at the Tent of Meeting.
2 Timothy 4:7
I have fought the good fight, I have finished the race, I have kept the faith.