Chabad.org — English translation + Rashi
Torah Temimah (Chullin 24a continued): the same “זאת” teaches that age does not disqualify priests the way it disqualifies Levites.
English: One might have thought that just as years disqualify Levites they also disqualify priests; therefore the Torah says “This is what pertains to the Levites” — this for Levites and not for priests.
Exclusion of priests from the age rule — the limiting word protects the priestly class from an age-based cutoff that applies only to Levites.
Priestly fitness is measured by lineage and physical integrity, not by a fixed retirement age of the kind imposed on the Levites in the wilderness.