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.
18 years of age for compulsory military service; 16 years of age for volunteers; 17 years of age for Law Enforcement Forces; 15 years of age for Basij Forces (Popular Mobilization Army); conscript military service obligation is 18 months; women exempt from military service (2012).8 years of age for voluntary military service; the armed forces law of 31 December 2004 did not specify a minimum age for enlistment, but the government claimed that no one younger than 18 was being recruited; mandatory retirement age 45 (enlisted), 50 (NCOs), and 55 (officers) (2012).