Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother's womb, and be born?
According to Arnold G. Fruchtenbaum, the ancient Jews had at least six experiences they referred to as being "born again," none of which applied to an old man such as Nicodemus (John 3:4). Each of the situations to which the Jews applied the term "born again" had to do with a drastic change or a new creation in one’s life.
The first of these was a person’s conversion to Judaism. When a person came up out of the waters of the mikveh, which was considered the womb of the world, he was considered born again and rendered a new creation. Upon his welcoming into the Jewish faith, he was said to be "under the wings of the Divine Presence." It is this designation that Boaz gave to Ruth when she converted to the faith (Ruth 2:12 ).
Other experiences that merited the phrase "born again" included being crowned King of Israel, being numbered with the believers of Israel at age thirteen (bar mitzvah), marriage, ordination as a rabbi, and becoming the head of a rabbinical school.
(Moseley, Ron. Yeshua: A Guide to the Real Jesus and the Original Church. Hagerstown, MD: Ebed Publications., pgs 131-132)