Parashas Bamidbar · Passuk 4:19
Do this with them, that they may live and not die when they approach the most sacred objects: let Aaron and his sons go in and assign every one of them, in turn, to his duties and to his porterage.
Chabad.org — English translation + Rashi
Source: Arachin 11b.
English: “Each man to his work” — from here [we learn] that one does not enter the service until he has grown and become a man.
In context: The doubled “ish ish” is read as requiring adult status before assuming the sacred burden.
Custom
Chazal derive that Levitical service begins only when one has reached full manhood.