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.
Army Warrant Officer: Be at least 18 years old at the time of enlistment and not have passed their 33rd birthday at the time of selection (for aviators) or their 46th birthday for all other specialities. Age waivers are considered on a case-by-case basis.