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.
Across the United States Armed Forces, the maximum age for enlistment for someone who has never served in the military before depends on the branch. For the Army, the maximum age is 35. For the Navy, age waivers start at 34-years-old. For the Air Force, the maximum age allowed to join is 39-years-old.