Chabad.org — English translation + Rashi
Torah Temimah: the age requirement (from twenty-five) applied only while the service involved carrying on the shoulder; once the Temple was stationary (Shiloh / permanent House), the age disqualification no longer operated in the same way.
English: One might have thought that the same rule applies at Shiloh and in the permanent House; therefore the Torah links it to “service of service and service of burden” — I spoke only of the time when the service was on the shoulder.
Historical limitation — the age rule is tied to the portable-sanctuary era.
Once the sanctuary ceased to be carried, the specific age-cutoff that protected the carriers no longer defined Levite eligibility in the same absolute way.