A mustang is slang term in the United States Armed Forces, referring to a commissioned officer who began his or her career as an enlisted service member. Mustangs are older and more experienced than their peers-in-grade who entered the military via commissioning from one of the service academies (such as the United States Merchant Marine Academy, United States Military Academy, United States Air Force Academy, United States Naval Academy, or United States Coast Guard Academy), Officer Candidate School, or the Reserve Officer Training Corps. During the Vietnam War, however, when some Army warrant officer pilots were offered a direct commission to 2nd or 1st Lieutenant, they were usually younger than 25 at the time of commission.