Which president was impeached twice?

Who was accused of assassinating John F. Kennedy? Which president survived an assassination attempt by John Hinckley Jr. in 1981? Which presidents have been impeached by the House of Representatives? Who was the first president to be impeached? Which president served the shortest time in office? Which president was the youngest ever elected? Which president was the youngest ever to take office, at age 42? Which president lived to age 100, the longest-lived in U.S. history?