If you are a man ages 18 through 25 and living in the U.S., then you must register with Selective Service. It’s the law. According to law, a man must register with Selective Service within 30 days of his 18th birthday. Selective Service will accept late registrations but not after a man has reached age 26. You may be denied benefits or a job if you have not registered. You can register at any U.S. Post Office or sss.gov.