John Gill Commentary Exodus 40:15

"and thou shalt anoint them, as thou didst anoint their father, that they may minister unto me in the priest`s office: and their anointing shall be to them for an everlasting priesthood throughout their generations." — Exodus 40:15 (ASV)

And you shall anoint them as you did anoint their
father
For Aaron was first anointed, and then his sons:

that they may minister to me in the priest's office ;
which they might not do before their anointing, though separated for it, and called to it:

for their anointing shall surely be an everlasting priesthood
throughout their generations ;
that is, this ceremony of anointing was to be used in all ages at the investiture of the high priest with his office, until the Messiah should come, who would put an end to the Aaronic priesthood; for not common priests, only the successors of the high priest, were anointed in later times; this present unction serving for all the priests that should follow in successive generations, so long as the priesthood continued.