Short Answer 
In the U.S., males are required by federal law to register with the Selective Service System at age 18 to prepare for potential military conscription. This mandate, established by the Selective Service Act, allows the government to draft individuals during national emergencies, and discussions are ongoing about possibly including females in the future.
Step 1: Understand the Requirement
In the United States, all males are legally required to register with the Selective Service System upon turning 18 years old. This obligation stems from federal law, specifically the Selective Service Act, which was established to prepare for potential military conscription. Understanding this requirement is essential for compliance and civic responsibility.
Step 2: The Role of Federal Law
The Selective Service System is not linked to individual states, meaning that all males across the nation must adhere to this federal mandate. This system was developed during World War I in 1917 and expanded prior to World War II in 1940 to ensure that the military had access to a pool of eligible individuals. Registration is crucial, as it enables the government to draft individuals if necessary during a national emergency.
Step 3: Future Implications of Registration
Currently, the Selective Service registration applies exclusively to males, but discussions have emerged concerning the inclusion of females. As of April 2016, the House Armed Services Committee proposed extending this requirement to women, signaling a potential change in the future. All males must register by the time they turn 18, ensuring their eligibility if conscription requirements arise.