The President is both the head of state and head of government of the United States of America, and Commander-in-Chief of the armed forces. Under Article II of the Constitution, the President is responsible for the execution and enforcement of the laws created by Congress.
You can join some branches of the United States military at the age of 38 years but not others. The Army, Navy, Air Force, Marine Corps and Coast Guard each has its own minimum and maximum age requirements for enlistees, including for new recruits in the active-duty ranks, the Reserves and the National Guard.
On October 16, 1940 all men between the ages of twenty-one and thirty-six were legally obligated to register for military service. After the U.S. entered the war, the draft age was lowered to eighteen, and local boards were instructed to choose first from the youngest.