In the United States, the Selective Service System requires almost all male U.S. citizens and immigrants, ages 18 through 25, to register with the government. If a military draft were to be reinstated, the Selective Service System would conduct a National Draft Lottery to determine the order in which young men would be drafted. The lottery would establish the priority of call based on the birth dates of registrants. The first men drafted would be those turning age 20 during the calendar year of the lottery. Men turning 21 in the year of the draft would be second priority, men turning 22 would be third, and so on until a man turns 26, at which time he is over the age of liability.

During World War I, all men aged 21 to 30 were required to enlist for military service for a service period of 12 months. The age limit was later raised in August 1918 to a maximum age of 45. The military draft was discontinued in 1920. During World War II, the age parameters for military drafts, established by the Selective Service Act, traditionally included males aged 18 to 25. However, these boundaries were expanded in periods of substantial conflict like World War II.

Currently, the age limit for military conscription is 25 years old. However, under a new policy change, active-duty Air Force and Space Force applicants can join up to the age of 42.