Young people have the power to dominate U.S. policy, but they’ve been sidelined for years. Will they vote in November? It depends. — An enormous voting bloc is about to come of age. By 2020, 27 percent of the U.S. electorate — some 54 million people — will be between the ages of 18 and 29. This group, comprising the trailing edge of millennials and the first crop of voting-age Gen Zers, represented…