This 13-year-old opened a bakery. For every cupcake he sells, he gives one to the homeless.

From a young age, Michael Platt loved two things: the Rev. Martin Luther King Jr. and cupcakes. He lingered by the “I Have a Dream” poster in his grandparents’ house, imagining ways he too could fight for justice. He memorized statistics about income inequality and childhood hunger. But he also …