Joan MacDonald’s health was in shambles at age 71 and her daughter, a fitness coach, said she needed to change. So she hit the gym and learned to balance her diet with the help of an iPhone. Now 75, she has a bodybuilder’s physique and 1.4 million followers on Instagram. She’s among a growing number of “grandfluencers," folks 70 and up, who have learned how to use social media and whose authenticity draws decades-younger fans. They bust myths about what’s possible in life’s sixth, seventh and eighth decades. Mae Karwowski of the influencer marketing agency "Obviously" has more than 100 influencers in her network between the ages of 60 and 80.