How old was Lincoln when he died?

Robert Lincoln graduated from Phillips Exeter Academy, then studied at Harvard University from 1861 to 1864 where he was a member of Delta Kappa Epsilon. (Later in life, Lincoln also joined the Delta Chi fraternity.) He then enrolled in Harvard Law School. However, he did not graduate and in 1865 joined the Union Army. He held the rank of Captain, serving in the American Civil War as part of General Ulysses S. Grant's immediate staff, in a position which sharply minimized the likelihood that he would be involved in actual combat.