when must all men register for the selective service?

All men must register for the Selective Service System between the ages of 18 and 26, ideally within 30 days of turning 18. Registration is required for almost all men who live in the United States, including U.S. citizens, immigrants, refugees, and even those living abroad who are U.S. citizens. Failure to register can lead to penalties including fines, prison, and loss of eligibility for federal jobs, student aid, and citizenship processes. Registration must be completed within 30 days after a male turns 18 and cannot be done after the 26th birthday, although late registration is accepted up until age 26.