Vincent's NT Word Studies
4. When he is old (gerwn wn). Literally, being an old man.Can he (mh dunatai). The interrogative particle anticipates a negative answer. Surely he cannot.
Second time. Nicodemus looks at the subject merely from the physical side. His second time is not the same as Jesus' anew. As Godet remarks, "he does not understand the difference between a second beginning and a different beginning."