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 earliest age at which individuals can enlist in any branch of the U.S. military is age 17, but at this age, they must have parental consent. However, the average age is higher given that some people enlist later. The maximum age for most branches is 35. On its recruiting website, the U.S. Army reported an average enlistment age of just under 21 in 2012. This was slightly lower than the average age from 2010 and 2011.