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.
Army. The Army is the oldest branch of the military and was established in 1775. The Army is considered to be the ground force of the military. Even though there are aviation units within the Army, their main missions are on the ground and the majority of the soldiers who serve have a job related to ground forces.