On Thursday Sept. 2, Jared Cohen walked out of the Truman Building with his luggage for a final time, after four years on the State Department’s Policy Planning staff, serving under both the Bush and Obama administrations. During his time in government, Cohen, who will be 29 in November, attracted much attention — both praise and controversy — for his unconventional thinking about statecraft: for calling on his friend Jack Dorsey to keep Twitter from going through with a scheduled maintenance shutdown during the heady days of the Iranian election last summer; for leading delegations of technology executives, including Google’s Eric Schmidt, to troubleshoot problems in Iraq; and for tweeting his observations, with a touch some critics found too lighthearted, to his 300,000-plus digital followers.