The original Selective Training and Service Act signed into law on 16 September 1940 set the age limits for both registration and liability for induction at at twenty-one to thirty-six. The original verbiage of the act with regards to age proved confusing, and a section was inserted in the Selective Training and Service Act on 13 November 1942 clarifying what was meant. The upper age limit was in fact those men who had not attained that age, so, i.e., twenty-one to thirty-six meant men who had attained their twenty-first birthday, but had not attained their thirty-sixth birthday.

On 16 August 1941, the ages of liability were modified so that men who had reached the age of twenty-eight by 1 July 1941, or by 1 July of any subsequent year, would not be inducted, or if in the service, could be released and transferred to the reserves for the period as specified in section 3 (c) of the original Selective Training and Service Act ("until he attains the age of forty-five, or until the expiration of a period of ten years after such transfer, or until he is discharged from such reserve component, whichever occurs first"). On 20 December 1941, with the outbreak of World War II, the men released under the August 1941 act were recalled to active duty and reassigned to their old units to the extent practicable, and the Selective Training and Service Act was amended to extend the ages of registration from twenty-one to thirty-five inclusive to eighteen to sixty-four inclusive, and the ages of liability for military service to twenty to forty-four years old inclusive. On 13 November 1942, the Selective Training and Service Act was amended to extend the ages of liability for military service to eighteen to forty-four inclusive; students who were eighteen or nineteen years old and were still in high school or college when ordered to report for induction in the last half of an academic year could have their induction postponed until the end of the year if they chose. On 9 July 1943, this amendment was amended to say that affected students could postpone their induction until the end of the academic year regardless of the date on which the year ended, or until they ceased to pursue a course of instruction, whichever date was the earlier.

On 5 December 1942, President Franklin D. Roosevelt signed Executive Order 9279, which terminated all voluntary enlistment (i.e., enlistment outside the controlling authority of the Selective Service System) for men aged eighteen to thirty-seven inclusive; this brought the personnel procurement of the Navy under the authority of Selective Service. Simultaneously, the armed forces declared that men thirty-eight or over were not acceptable for induction, making the ages of liability eighteen to thirty-seven inclusive. The restrictions on voluntary enlistment were removed by President Harry S. Truman's Executive Order 9605, signed on 29 August 1945. Immediately after the cessation of hostilities with Japan, President Truman decreed that men aged twenty-six or over would be deferred from induction

The eighteen to twenty-five age bracket remained in effect until 14 May 1946, when Congress passed Public Law 379, which amended the Selective Training and Service Act so that the ages of liability for military service were changed to twenty to twenty-nine inclusive. The verbiage of the act meant that, technically, men up to the age of thirty-five years and seven months old could be drafted, but the military did not have any intention of actually inducting them:

"SEC. 3 (a) Except as otherwise provided in this act, every male citizen of the United States, and every other male person residing in the United States, who is between the ages of 20 and 30, at the time fixed for his registration, or who attains the age of 20 after having been required to register pursuant to section 2 of the Act, shall be liable for training and service in the land or naval forces of the United States."

The statement from President Truman being;

It will be noted that the Congress has restricted inductions, except of volunteers, to the age group who become 20 or who are between 20 and 30 at the time fixed for their registration. Seemingly, it was the intent of Congress to include only those who are now under 30, but the clear words of the law include as liable all men born on or subsequent to October 17, 1910, who have reached the age of 20. Thus, men up to 35 years, 7 months of age could be drafted.

On 29 June 1946, the Selective Training and Service Act was amended to change the ages of liability for military service to nineteen to forty-four years old inclusive; eighteen year olds were still required to register. The Secretary of War requested successfully that President Truman change the ages of liability to nineteen to twenty-nine inclusive, an age bracket which remained in effect until 11 October 1946, when it was announced that inductions scheduled on or after 16 October 1946 would be suspended. The last provisions of the Selective Training and Service Act expired without further action by Congress on 31 March 1947.

Source:

United States. Selective Service System. Age in the Selective Service Process, Special Monograph No. 9. Edited by Preston W. Hale. Washington, D.C.: United States Government Printing Office, 1946.