Marc Bernstein was in the third grade when the U.S. was attacked on 9/11. He was a high school student when the U.S. economy dipped during the Great Recession. Today, as the U.S. faces the Covid-19 pandemic, the 27-year-old Bernstein is leading a company of nearly 40 employees through the crisis. Bernstein, who leads software startup Balto, is one of several 20-something CEOs in St. Louis’ flourishing entrepreneurship sector who are facing the first major challenges of their careers…