Joan MacDonald’s health was suffering. At age 71, she was overweight and on several medications. She had high cholesterol, rising blood pressure and kidney trouble. Her daughter is a fitness trainer. She warned that MacDonald would become severely unwell if she did not make big changes. So she did. She went to the fitness center for the first time and learned to eat healthy with the help of a new tool: an iPhone. Now age 75, MacDonald promotes fitness on social media. She has 1.4 million followers on Instagram. She is part of a growing number of “grandfluencers” people over the age of 70 who have huge numbers of social media followers. Many of their fans are several years younger. “It’s so rare to find someone her age being able to do all these things,” said 18-year-old follower Marianne Zapata. MacDonald and other older social media influencers are turning their popularity into a profit. MacDonald has paid partnerships, including with the sportswear and nutrition company Women’s Best. She also just launched her own health and fitness app.