Jamieson, Fausset and Brown on John 8:57-59

57-59. Then said the Jews unto him, Thou art not yet fifty years old--"No inference can be drawn from this as to the age of our Lord at the time as man. Fifty years was with the Jews the completion of manhood" [Alford].

and hast thou seen Abraham?--He had said Abraham saw Him, as being his peculiar privilege. They give the opposite turn to it--"Hast Thou seen Abraham?" as an honor too great for Him to pretend to.