The law states that any male who is in the US legaly between 18 and 26 must register with selective service.  It is not just the citizenship application that requires Selective Service registration, other government agencies like department of education requires you to be registered for selective service in order for you to receive federal financial aid.  If you are less than 18 years old or over 26 years old then you are fine and don’t have to worry about registering for the Selective Service. usa citizenship