Chabad.org — English translation + Rashi
Torah Temimah notes the apparent tension with the “from thirty years” language elsewhere and the Rambam’s ruling that for permanent generations a Levite enters service when he becomes an adult (“ish”).
English: “From twenty-five years old” — the desert standard. For subsequent generations the operative criterion becomes majority / physical maturity rather than a fixed twenty-five-year threshold.
Distinction between desert and permanent eras — the numerical age is era-specific.
The Torah’s age numbers describe the conditions of the traveling camp; later halakhah adapts the principle to a stationary sanctuary.