32 CFR 1627.1: Who may volunteer.

Any registrant who has attained the age of 17 years, who has not attained the age of 26 years, and who has not completed his active duty obligation under the Military Selective Service Act, when inductions are authorized, may volunteer for induction into the Armed Forces unless he;

(a) Is classified in Class 4-F or is eligible for Class 4-F; or

(b) Has been found temporarily unacceptable with reexamination believed justified (RBJ) and the period of time specified for his return for examination has not been terminated and the basis for his temporary rejection continues to exist; or

(c) Is an alien who has not resided in the United States for a period of at least one year; or

(d) Has not attained the age of 18 years and does not have the consent of his parent or guardian for his induction.