One Year Difference

I started reading a book review on religion when I noticed the reviewer's info:

Mark Oppenheimer edits The New Haven Review and is the author of “Thirteen and a Day: The Bar and Bat Mitzvah Across America.”

Bar Mitzva is at age thirteen.

Bat Mitzva, though, is at age twelve.

So That's the Reason for a Bar-Mitzvah

I am told that the Barna Group, a prominent Christian demographic polling organization, concluded from its surveys that in essence what you believe by the time you are thirteen is what you will die believing.

Source