Man must be born again of water and the spirit. We would say, at Baptism (and Confirmation, which is the other half of what is generally meant by Adult Baptism).
But of course there has been an awful lot of additional 'gloss' added to that short explication.
The misunderstanding of the ‘born again’ is the statement of Nicodemus, not Jesus. Jesus made haste to correct him on the spot.