Parashas Bamidbar · Passuk 4:03
from the age of thirty years up to the age of fifty, all who are subject to service, to perform tasks for the Tent of Meeting.
Chabad.org — English translation + Rashi
Source: Chullin 24a.
English: “From thirty years old” — and elsewhere it says from twenty-five years old; how is this? Twenty-five for training, thirty for service.
In context: The apparent contradiction between 25 and 30 is resolved by distinguishing the period of apprenticeship from the full work age.
Custom
Chazal reconcile the two ages: five years of training from 25, full service from 30.