Joan MacDonald was 71 and in poor physical shape when her fitness coach daughter told her that she needed to make changes or have her health deteriorate. She began watching what she ate and working out. Four years later she has 1.4 million followers on Instagram who check in to get advice and see what Ms. MacDonald is up to. She has become a “grandfluencer,” a small but growing list of people over 70 who are inspiring others via social media.