Nicodemus answered him, "How can these things be?"
How can a man be born again when he is old ... ?  Part of the answer to that question, namely the reception of the Holy Spirit, was not even available to the apostles at the time Nicodemus made this inquiry; and his mistake of waiting until he had all the answers was tragic.  His greater concern should have been the acceptance of what he knew, namely, that he should have accepted the baptism of John.  If he had submitted to that, as did Christ and the apostles, he would have experienced more, in time, and would have been truly "born again."