Olivia Newton-John is one of those celebrities who without expending any effort whatsoever, can induce an entire generation of people to revert to their adolescent selves. On April 23, 2016, she did just that when she took the stage at Montclair, New Jersey’s Wellmont Theater. With a tight band and no frills approach—zero lights or stage production, just hits and multiple glittering shawls over a smart black ensemble—the still-comely Ms. John did not skimp on her pop nuggets as she led the crowd in what amounted to a group singalong. The highlight of the show for most was a Grease medley that recruited Didi Conn from the audience to create some new Pink Ladies memories for young and old. For…