John 8:57

Thou art not yet fifty yearn old,— Understanding what our Lord laid in a natural sense, the Jews thought he affirmed that he lived as man in the days of Abraham; which they considered as ridiculous, he not being yet fifty years of age: for they had no conception of his Divinity, though he had told them several times that he was the Son of God. Christ was not now five and thirty; but Erasmus thinks, that, worn with labours, he might appear older than he was. Lightfoot imagines, that as the Levites were discharged from the temple service at fifty, (Numbers 4:3; Numbers 4:23.) that age was proverbially used; as it certainly might have been without any such institution relating to them, it being usual among most nations to express themselves on such occasions by some round number.