NEW YORK — That 59-year-old Marc Jacobs has become an influencer, speaking directly to his 1.7 million Instagram followers about everything from relationships to plastic surgery, is perhaps no surprise. Many of his peers — including Isaac Mizrahi and Todd Oldham — have also attracted fervent online fans, especially those nostalgic for the 1990s, when fashion designers were idolised like rock stars.
But Jacobs is different.
His followers…