At what age must all men register for the Selective Service?

Men are required to register for the Selective Service at the age of 18. This registration is a system used by the U.S. government to maintain a list of individuals who could be drafted into military service if necessary. While men can pre-register as early as 17 years old, they must complete the registration process by their 18th birthday. This requirement is part of the legal obligations established by the Military Selective Service Act, which aims to ensure that the country is prepared for national defense scenarios. Therefore, the answer of 18 is accurate as it reflects the legal age at which registration must occur.