A.T. Robertson Commentary John 3:4

"Nicodemus saith unto him, How can a man be born when he is old? can he enter a second time into his mother`s womb, and be born?" — John 3:4 (ASV)

Being old (γερων ων). Nicodemus was probably familiar with the notion of re-birth for proselytes to Judaism for the Gentiles, but not with the idea that a Jew had to be reborn. But "this stupid misunderstanding" (Bernard) of the meaning of Jesus is precisely what John represents Nicodemus as making. How "old" Nicodemus was we do not know, but surely too old to be the young ruler of Lu 18:18 as Bacon holds. The blunder of Nicodemus is emphasized by the second question with the μη expecting the negative answer. The use of δευτερον adds to the grotesqueness of his blunder. The learned Pharisee is as jejune in spiritual insight as the veriest tyro. This is not an unheard of phenomenon.