In the United States post-Pearl Harbor, it was made mandatory for every male aged 18 to 64 (from the 18th birthday to the last day of the 64th year of life) to register for the draft through the Selective Service, with men ages 18 to 37 being subject to it.

Draft Registrations:

Registration	Date	Birth dates	Note
1	Oct. 16, 1940	Oct. 17, 1904-Oct. 16, 1919	Men 21-35 years old
2	Jul. 1, 1941	Oct. 17, 1919-Jul. 1, 1920	Men who had turned 21 since the last registration, and men who were of age but had failed to register in the first registration
3	Feb. 16, 1942	Feb. 17, 1897-Oct. 16, 1904 and Jul. 2, 1920-Dec. 31, 1921	Men 20-21 and 37-44 years old
4	Apr. 27, 1942	Apr. 28, 1877-Feb. 16, 1897	Men 45-64 years old (not liable for military service)
5	Jun. 30, 1942	Jan. 1, 1922-Jun. 30, 1924	Applicable 18 year old men
6 (I)	Dec. 11-17, 1942	Jul. 1-Aug. 31, 1924	Applicable 18 year old men
6 (I)	Dec. 18-24, 1942	Sept. 1-Oct. 31, 1924	Applicable 18 year old men
6 (I)	Dec. 26-31, 1942	Nov. 1-Dec. 31, 1924	Applicable 18 year old men
6 (II)	Jan. 1, 1943-Mar. 31, 1947	Jan. 1, 1925-Mar. 31, 1929	Men were to register as they turned 18 years old
Extra	Nov. 16-Dec. 31, 1943	Jan. 1, 1899-Dec. 31, 1925	Men 18-44 years old living abroad. Beginning Jan. 1, 1944, men were to register as they turned 18 years old
Special	Oct. 23-31, 1944	Nov. 1, 1899-Oct. 31, 1926	Men 18-44 years old living in the US Virgin Islands. Beginning Nov. 1, 1944, men were to register as they turned 18 years old

That does not mean that every registered male would have served, however. Men who were morally, physically, or mentally incapable of service as decided by doctors and psychiatrists, or held important positions in wartime society such as farming, working at defense plants, or officiating religion, were exempt from service. Initially, men who had children were exempt, but this provision was eliminated on October 1, 1943; after this date, men whose children were born before December 7, 1941 were drafted like anyone else, unless they were the sole caregiver in the household.

On December 5, 1942, President Franklin D. Roosevelt signed Executive Order 9279, which among other things, barred men who were 18 to 37 from voluntarily enlisting in the military; men of these ages could now only join the military by being drafted. The order was repealed on September 7, 1945.

Men who were 38 to 44 years old also had to register for the draft, but would not actually be drafted due to their age; they could still join the military voluntarily if they met the standards for enlistment. If an emergency situation arose, such as a war for national survival, these men were the next in line to be drafted; you will notice that in the "crisis" years of the war, (1941-1942) the age requirement for an age deferment was quite high, and was lowered as the war went on. A similar situation existed for 17 year olds; they, in particular, did not have to register for the draft, but could join the military at their own volition. Those men from 45 to 64 years old were exempt entirely from any kind of military service, but were registered in order to get a rough idea of how many war industry workers the United States had; their registration card included a section for employment and personal skills.

During WWII, women could also serve in the US military (but in non-combat positions such as nurses or ferry pilots) but they had to enlist voluntarily; they were not drafted, nor compelled to register for it.

Selective Service Classifications 1940-1947:

Classification	Info
I	Available for military service
I-A	Available for military service
I-A (B)	Acceptable for military service, below standards for general military service
I-A (L)	Available for limited military service
I-A-O	Available for noncombatant military service (conscientious objector)
I-A-O (B)	Acceptable for noncombatant military service, below standards for general military service
I-A-O (L)	Available for noncombatant limited military service
I-A (remanded)	Available for military service after correction of medical defects
I-A-O (rem.)	Available for noncombatant military service after correction of medical defects
I-B	Available for limited military service
I-B-O	Available for noncombatant limited military service
I-B (rem.)	Available for limited military service after correction of medical defects
I-B-O (rem.)	Available for noncombatant limited military service after correction of medical defects
I-C (Ind.)	Inducted member of armed forces
I-C (Enl.)	Enlisted member of armed forces
I-C (Disc.)	Discharged (honorably) from the armed forces
I-C (Dec.)	Deceased while in Class I-C
I-D	Deferred student, fit for general military service
I-D-O	Deferred student, fit for general noncombatant military service
I-E	Deferred student, fit for limited military service
I-E-O	Deferred student, fit for limited noncombatant military service
I-G	Member of or honorably separated from armed forces of cobelligerent nation (later extended to include registrants separated from American Field Service or Merchant Marine, and persons interned by an enemy nation
I-H	Available for military service but deferred due to age; 11-18-42: age 45 and over; 10-5-1944: age 38 and over; 8-31-1945: age 26 and over; 7-17-1946: age 30 and over
II	Deferred from military service because of occupation
II-A	Deferred in support of national health, safety, or interest
II-A (L)	As II-A, but found to be qualified for potential limited military service
II-A (F)	As II-A, but rejected for potential military service
II-B	Deferred in support of war production
II-B (L)	As II-B, but found qualified for potential limited military service
II-B (F)	As II-B, but rejected for potential military service
II-C	Deferred in agriculture
II-C (L)	As II-C, but found qualified for potential limited military service
II-C (F)	As II-C, but rejected for potential military service
III	Deferred from military service because of dependency
III-A	Deferred for dependency reasons [eliminated 10-1-43; reestablished 11-15-45]
III-B	Deferred both by reason of dependency and occupation essential to the war effort
III-C	Deferred both by reason of dependency and agricultural occupation
III-D	Deferred by reason of extreme hardship and privation to wife, child, or parent
IV	Deferred from military service because unacceptable or exempt
IV-A	Man who has completed service [applicable in time of peace only]
IV-A (H)	Man found unfit for military service, later deferred by reason of age; 11-18-42: age 45 and over; 10-5-44: age 38 and over; 8-31-45: age 26 and over [this version of the classification was used in wartime]
IV-B	Official deferred by law
IV-C	Alien not acceptable to armed forces and certain neutral aliens
IV-D	Minister of religion or divinity student
IV-E	Conscientious objector, available for or assigned to work of national importance
IV-E (B)	Conscientious objector, under 26 years of age, acceptable under lowered physical standards for work of national importance
IV-E (H)	Conscientious objector deferred by reason of age (over age 27)
IV-E (L)	Conscientious objector qualified for limited service
IV-E (LS)	Conscientious objector who would otherwise be in Class I-D or I-E available for limited service in civilian work of national importance
IV-E (S)	Conscientious objector who would otherwise be in Class I-D or I-E
IV-E (Disc.)	Conscientious objector separated from work of national importance by issuance of a Certificate of Discharge
IV-E (Sep.)	Conscientious objector separated from work of national importance other than by issuance of a Certificate of Discharge or of Death
IV-E (Dec.)	Deceased while in Class IV-E
IV-F	Rejected for military service because of physical, mental, or moral reasons
IV-H	Man found unfit for military service, later deferred by reason of age; 7-17-1946: age 30 and over [this version of the classification was used in peacetime]