Final answer:
All men ages 21 to 30 years old could be drafted to serve in the armed forces under the Selective Service Act during various conflicts like World Wars and the Civil War, with service terms varying by legislation.
Step-by-step explanation:
All men ages 21 to 30 years old could be drafted to serve in the armed forces for up to the duration of the conflict or specific terms decided by the draft legislation at various points in history.
During World War I, the Selective Service Act of 1917 required all men aged twenty-one through thirty to register for the draft, a range which was extended in 1918 to include men between eighteen and forty-five. This act led to the creation of the American Expeditionary Force (AEF), with 2.8 million of the over 24 million men drafted serving under General John J. Pershing.
Congress passed the Enrollment Act in March 1863 during the Civil War, which required registration and potential conscription of men based on their age and marital status. Different approaches to the draft were also taken in other conflicts and by various states such as North Carolina during the Revolutionary War.
Furthermore, established in 1940 during WWII, the Selective Service System was also used to draft men during the Vietnam War, focusing on men aged 18-26. Over the course of WWII, approximately fifty million men registered for the draft, with ten million being inducted into the service.