זאת אשר ללוים. שָׁנִים פּוֹסְלִים וְאֵין הַמּוּמִין פּוֹסְלִים בָּהֶם (ספרי; חולין כ"ד): זאת אשר ללוים THIS IS THAT WHICH BELONGETH TO THE LEVITES [FROM TWENTY FIVE YEARS etc.] — This suggests: years (the question of age) can disqualify them for the service (i.e. if they are younger than 25 and older than 50 years), but bodily blemishes cannot disqualify them, [as in the case of priests] (Sifrei Bamidbar 62; Chullin 24a).
מבן חמש ועשרים. וּבְמָקוֹם אַחֵר אוֹמֵר "מִבֶּן שְׁלֹשִׁים שָׁנָה" (במדבר ד'), הָא כֵיצַד? מִבֶּן כ"ה בָּא לִלְמֹד הִלְכוֹת עֲבוֹדָה וְלוֹמֵד חָמֵשׁ שָׁנִים וּבֶן שְׁלֹשִׁים עוֹבֵד, מִכָּאן לְתַלְמִיד שֶׁלֹּא רָאָה סִימָן יָפֶה בְּמִשְׁנָתוֹ בְּחָמֵשׁ שָׁנִים שׁוּב אֵינוֹ רוֹאֶה (ספרי; חולין כ"ד): מבן חמש ועשרים FROM TWENTY AND FIVE [YEARS] AND UPWARD [THEY SHALL GO IN TO WAIT UPON THE SERVICE] — But in another passage, it states, (Numbers 4:3) “from thirty years old [and upwards … all that enter into the host to do the work in the tabernacle]”! How are these apparently contradictory passages to be reconciled? In the following way: From twenty five years on he (the Levite) comes to learn the laws regulating the service, and he studies for five years, and at the age of thirty he may actually do the service. — From here we may derive the principle that a pupil who does not see an auspicious omen (i. e. success) in his study during five years will never see it. (Sifrei Bamidbar 62; Chullin 24a.)