John Lightfoot on Luke 1:18

[For I am an old man.] If so old a man, why then was he not sequestered from the service of the Temple by the law of superannuation? Numbers 4:3, 8:24,25. Hear what the Rabbins say in this case:

"There is something that is lawful in the priests, that is unlawful in the Levites: and there is something lawful in the Levites, that is unlawful in the priests. The Rabbins deliver; the priests upon any blemish are unfit; as for their years they are not unfit; the Levites for their years may be unfit, but by reason of blemish are not. From that which is said, that at the age of fifty years they shall cease waiting, we learn that years may make the Levites unfit. Perhaps the priests also are made unfit through years: and indeed, does it not seem in equity, that if the Levites, whom a blemish doth not make unfit, should yet be made unfit by superannuation, should not much more the priests be made unfit by superannuation, when even a spot or blemish will make them unfit? But the text saith, This is the law of the Levites; not, This is the law of the priests. The Rabbins deliver: What time a priest comes to maturity, till he grow old, he is fit to minister; and yet a spot or blemish makes him unfit. The Levite from his thirtieth to his fiftieth year is fit for service; but being superannuated, he becomes unfit. How must this be understood concerning the Levites? To wit, for that time wherein the ark was in the wilderness: but at Shiloh and in the Temple they were not rendered unfit, unless through the defect of their voice."