Male residents of the U.S. between the ages of 18 and 25 must register with the Selective Service System, even if they are not citizens. In 2019, Congress considered a proposal to expand Selective Service registration to women, but so far the law has not changed. The Selective Service System provides the government with the names of people who are eligible to be drafted into the armed forces if a national emergency arises. The requirement does not apply if the foreign national was covered by a non-immigrant visa throughout their time in the U.S. between the ages of 18 and 25. It does apply to foreign nationals who do not have legal status in the U.S. Failing to comply with the Selective Service requirement can prevent you from receiving citizenship. If you applied for a green card between the ages of 18 and 25, Form I-485 gives USCIS permission to give your information to the Selective Service System, but you cannot rely on USCIS to do this for you.
If you have not yet turned 26, you can register for the Selective Service now to comply with the requirement. You can register by mail or online. Otherwise, you cannot register for the program, and you may need to accumulate five years of good moral character after you turned 26 to overcome this bar to citizenship. If you are married to a U.S. citizen, you may need to accumulate only three years of good moral character. In other words, you can apply for citizenship once you turn 31 or possibly once you turn 29.
Proof of Innocent Failure to Register
Some foreign nationals may want to become citizens without waiting until they turn 31 (or 29). Since the bar applies only to foreign nationals who willfully failed to register for the Selective Service, they may be able to get citizenship sooner if they can prove that they unintentionally failed to sign up because they did not know that it was required. With Form N-400, they would need to submit a Status Information Letter from the Selective Service System that explains that they no longer need to register because they are over 25. They also would need to provide a declaration under oath regarding their reasons for failing to register.
This declaration may need to explain why the applicant did not register for the Selective Service if they were told about the requirement in high school. They might be able to argue that they thought that the requirement was limited to U.S. citizens. However, if you received your green card in 2001 or later, you may not be able to claim ignorance of the rule because you should have received information about the registration requirement at that time.
You can supplement your declaration with declarations from teachers and other authority figures. They may be able to corroborate your explanation and provide additional reasons for why you did not know that you needed to register. Again, you do not need to worry about your failure to register unless you are trying to become a citizen before turning 31 (or 29). You may find it easier to simply wait until then rather than compiling these supplementary documents.