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.
By the guidelines set down by the Selective Service Act, all males aged 21 to 30 were required to register for military service. At the request of the War Department, Congress amended the law in August 1918 to expand the age range to include all men 18 to 45, and to bar further volunteering.