SUMMARY: This Directive establishes basic entrance qualification standards for enlistment, appointment, and induction into the Armed Forces in accordance with Section 113 of Title 10, United States Code and delegates the authority to specify certain of those standards to the Secretaries of the Military Departments. This Directive establishes the age, citizenship, education, aptitude, physical fitness, dependency status, moral character, and other disqualifying conditions that are causes for refection for military service. Other standards my be prescribed in the event of mobilization or national emergency. It sets standards designed to ensure that individuals under consideration for enlistment, appointment, or induction are able to perform military duties successfully, and to select those who are the most trainable and adaptable to Service life.
There's a document there you can download (unless you're paranoid).