Final answer:
Truman's reference to the last sixteen years in his address encompasses key historical events including the Great Depression, Roosevelt's presidency, and World War II, acknowledging their impact on American society and global standing.
Step-by-step explanation:
President Truman's reference to the last sixteen years in his 1949 State of the Union address is significant because it encapsulates a transformative period in American history. This era includes the repercussions of the Great Depression, the unprecedented changes during Franklin D. Roosevelt's presidency, and the global upheaval of World War II. Truman likely referred to this period to acknowledge the legacy of challenges and triumphs that shaped the nation, such as the expansion of executive power, establishment of the New Deal, economic recovery, and the wartime efforts which repositioned the United States as a global power.