When must all men register for the Selective Service?

All men are required to register for the Selective Service at the age of eighteen. This registration is a part of the United States' preparedness for potential military conscription in the event of a national emergency or war. The requirement holds true for U.S. citizens and immigrant males living in the country.

Registering at eighteen allows the government to maintain an updated database of eligible individuals who could be drafted, should the need arise. Although individuals can begin the registration process any time after they turn sixteen, the formal requirement to register is specifically set at age eighteen. Thus, the correct response reflects the legal obligation and the age established by federal law.