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.
For reserve duty... The maximum age to enlist is age 39. For a reserve commission, the minimum age is 21 and maximum age is 36 years old; however, a waiver may be granted for applicants with prior military service (up to age 39). The Officer Candidate School (OCS) minimum age is 21 and maximum age is 26 years old. 2  However, applicants with prior active duty service in any military branch may exceed this age limit by the number of months of service (not to exceed age 31).