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.
In general, the Defense Department restricts enlistment to those 35 and younger. Prior enlisted service members can subtract their previous years of service from their age in order to extend eligibility. For example, the current age cutoff in the Air Force is 27.