from twenty:

In "From thirtyª yearsª oldª¹ and upwardª even until¹ fiftyª yearsª old,ª all¹ that enterª° into the host,ª to doª° the workª in the tabernacleª of the congregation.ª" {Num 4:3}, the Levites are appointed to the service of the tabernacle at the age of 30 years; and in "These¹ [were] the sonsª of Leviª after the houseª of their fathers;ª [even] the chiefª of the fathers,ª as they were countedª° by numberª of namesª by their polls,ª that didª° the workª for the serviceª of the houseª of the LORD,ª from the ageª¹ of twentyª yearsª and upward.ª" {1Ch 23:24}, they are ordered to commence their work at 20 years of age. In order to reconcile this apparent discrepancy, it is to be observed:

1. At the time of which Moses speaks in "From thirtyª yearsª oldª¹ and upwardª even until¹ fiftyª yearsª old,ª all¹ that enterª° into the host,ª to doª° the workª in the tabernacleª of the congregation.ª" {Num 4:3}, the Levitical service was exceedingly severe, and consequently required full grown, robust men, to perform it; the age of 30 was therefore appointed as the period for commencing this service, the weightier part of which was probably there intended.
2. In this place God seems to speak of the service in a general way. Hence the age of 25 is fixed.
3. In David's time, and afterwards, in the fixed tabernacle and temple, the laboriousness of the service no longer existed, and hence 20 years was the age appointed. "From thirtyª yearsª oldª¹ and upwardª even until¹ fiftyª yearsª old,ª all¹ that enterª° into the host,ª to doª° the workª in the tabernacleª of the congregation.ª" {Num 4:3} + "From thirtyª yearsª oldª¹ and upwardª until¹ fiftyª yearsª oldª shalt thou numberª° them; all¹ that enter inª° to performª° the service,²¹ to doª° the workª in the tabernacleª of the congregation.ª" {Num 4:23} + "Now the Levitesª were numberedª° from the ageª¹ of thirtyª yearsª and upward:ª and their numberª by their polls,ª man by man,ª was¹ thirtyª and eightª thousand.ª" {1Ch 23:3} + "These¹ [were] the sonsª of Leviª after the houseª of their fathers;ª [even] the chiefª of the fathers,ª as they were countedª° by numberª of namesª by their polls,ª that didª° the workª for the serviceª of the houseª of the LORD,ª from the ageª¹ of twentyª yearsª and upward.ª ... For¹ by the lastª wordsª of Davidª the Levitesª¹ [were] numberedª from twenty² years² oldª¹¹¹ and above:ª" {1Ch 23:24-27} + "And the patternª of all¹ that¹ he had¹ by the spirit,ª of¹ the courtsª of the houseª of the LORD,ª and of all¹ the chambersª round about,ª of the treasuriesª of the houseª of God,ª and of the treasuriesª of the dedicated things:ª ... Also for the coursesª of the priestsª and the Levites,ª and for all¹ the workª of the serviceª of the houseª of the LORD,ª and for all¹ the vesselsª of serviceª in the houseª of the LORD.ª" {1Ch 28:12-13}