Chabad.org — English translation + Rashi
Torah Temimah cites Chullin 24a: “זאת אשר ללוים” teaches that age-disqualification applies only to Levites, not to priests; the word “זאת” excludes other categories.
English: It was taught: one might have thought that just as Levites are disqualified by age they are also disqualified by blemishes; therefore the Torah says “This is what pertains to the Levites” — this for Levites and nothing else for Levites.
Gezerah / limiting expression — “זאת” confines the age rule (and excludes blemish-disqualification) to Levites alone.
The age limits that govern Levite service do not automatically transfer to the priesthood; each class has its own criteria of fitness.