Chabad.org — English translation + Rashi
Torah Temimah (Chullin 24a / Yerushalmi): from fifty the Levite returns from the service of the work.
English: “And from fifty years old he shall return from the service of the work and shall serve no more” — we learn that for the Levites the years disqualify them.
Age disqualification — the numerical limit on Levite service is stated in the plain text.
The Levite’s heavy service has a defined end; the Torah itself sets the retirement threshold.