Too Old

The possuk says (8:25) regarding the service of the לוים in the mikdash, ומבן חמישים שנה ישוב מצבא העבודה ולא יעבד עוד.  Rashi and Ramban explain that they are no longer fit for carrying the mishkan but can do the other avodat hamikdash.  The Gemorah in Chullin 24 has a derasha that this law of becoming unfit for the avodah only applied in the midbar but for future generations they are fitting until their voice becomes uncapable of singing.  This halacha can be viewed in two ways, either it is a siman that the age of the pesul of the לוים in the Beis Hamikdash is determined by his voice or the lack of a sweet voice is the סיבת הפסול.  The Rambam says (Kli Mikdash 3:8) כשיתקלקל קולו מרוב הזקנה יפסל.  The Rambam understands that the lack of voice is a simam of determining old age but isn’t a pesul in it of itself.  It would come out according to him that if a younger levi’s voice became sour then he would remain fit for the avodah.  On the other hand, Rashi Chullin 24b says the pesul is because the leviem must be able to sing on the same note and by that logic any levi who’s voice isn’t up to par would become disqualified to serve.

The Rambam adds שירה אבל יהיה מן השוערים יראה לי שאינו נפסל אלא לומר.  This would be modeled after the pesul of above 50 years old in the midbar where the levi is still fit for other avodot (Mahari Kurkos.)  This comparison only makes sense assuming that the pesul of קול is also because of becoming too old to serve in the Mikdash; we would have thought the levi is considered to be too blemished to serve at all,קמ"ל he is still fit to close the doors.  However, according to Rashi that it’s a pesul because of the lack of a sweet voice, this is obvious that the pesul would be limited only to singing (see Masseh Yad and additions to the footnotes on Rashba Chullin by Rav Yaakov Elon.)