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.
US History Trivia: What is the oldest branch of the U.S. Military? It's a bit strange to think about, but the five American military branches (Army, Navy, Air Force, Marines and Coast Guard) were not all actually formed at the same time (okay, maybe it's not too strange to think about, at least in terms of the Air Force!).