From Numbers 4:
1 And the Lord spake unto Moses and unto Aaron, saying,
2 Take the sum of the sons of Kohath from among the sons of Levi, after their families, by the house of their fathers,
3 From thirty years old and upward even until fifty years old, all that enter into the host, to do the work in the tabernacle of the congregation.
The thirty to fifty age range is repeated multiple times over the course of the chapter, and has been understood to be the origin of the custom that a man was expected to be 30 years of age to begin a ministry (including an application in reckoning the chronology of John the Baptist and Jesus).
Sometimes Jewish chronology counts inclusively -- e.g. "the 30th year" is the time between one's 29th birthday and 30th birthday. Sometimes they do not count inclusively. In this sense "30 years of age" could refer to 29 years old or to 30 years old.
Is there any indication that the work of the ministry began in the 30th year vs at age 30? And that retirement took place at age 49 vs age 50? (or even 51)
Addendum for clarification
This question is not about the Numbers 8:24 reference to duties at age 25, though in theory the same question could be asked of it--whether 25 means 25th year (24 years old), or 25 years old.