In 1973, the draft ended and all four branches of the U.S. military converted to an all-volunteer force. Enlistment now means filing an application to join with a military recruiter, who has the responsibility to screen applicants and carry out the necessary background checks. Basic requirements are similar across all branches, with minor variations, and enlistees can be rejected for not meeting age, legal residency and other guidelines.

Citizenship and Residency

You must be a U.S. citizen or legal resident to join the military. You must provide proof of status to your recruiter, who will reject your application, with no exceptions, if you don't meet this requirement. The military does not assist with immigration matters, which are handled by the U.S. Citizenship and Immigration Services, a different branch of the federal government.

Age and Education

The military will reject your application if you are too young or too old. You must be at least 17 years of age to enlist in any service branch; if you are 17, you must have your parents' consent. The upper age limit varies: 27 for the Air Force, 34 for the Army and Navy, and 29 for the Marines. All service branches require a high school diploma or GED.

Aptitude Tests

If you fail to make a passing grade on the Armed Services Vocational Aptitude Battery, your application will not succeed. You take these tests at Military Entrance Processing Stations or satellite Military Entrance Test sites. The categories include general science, mathematics knowledge, paragraph comprehension and mechanical comprehension. Test scores are given in percentiles from 1 to 99; the passing scores vary for the different branches: 50 for the Air Force, 31 for the Army, 50 for the Navy and 32 for the Marines.

Physical Condition and Criminal Record

The military generally rejects applicants with criminal convictions on their record. The military has access to all criminal records, from felonies all the way down to traffic tickets, and recruiters will apply their own standards to any history they find, even if the record has been officially expunged. In addition, applicants may fail a security check known as the Entrance National Agency Check, in which the service queries federal agencies, such as the FBI, for any investigative files on potential recruits.