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.
Traditionally, most people join the U.S. military in their late teens or early to mid-twenties. In 2011 the average enlistment age for people joining the Army was 21.3 years. Each of the four military service branches and the Coast Guard also have their own maximum age standards for enlistment or officer commissioning. So while the maximum military enlistment age set by Congress is 42 years, if you're 35 or older you might have a tougher time trying to enlist.