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.
The Army is also the largest U.S. Military Service. There are approximately 76,000 officers and 401,000 enlisted members in the active duty Army. The Army is supported by two Reserve Forces which can be tapped for trained personnel and equipment during times of need: The Army Reserves, and the Army National Guard.