Craig C. Martin is only the fourth person to serve as chair of the 105-year-old Jenner & Block LLP. At age 55, Martin is also the youngest person to hold that title.“I think it’s a signal that we are looking toward the future,” Martin said. “There is a new generation of leadership at Jenner — my appointment as chair reflects that.”Martin succeeds Anton R. Valukas, who stepped down as firm chair in 2017. Albert E. Jenner was the first firm chair, followed by Jerold S. Solovy.The firm …