Being 26 to 31 years of age, you are technically outside the age of registering for Selective Service. That being said, you still need to prove to USCIS that you are in good moral standing to become a U.S. Citizen. Speak with your immigration lawyer about your options if you are applying for naturalization after your turn 26.
As a rule of thumb, you need to show at least five years of good moral character. Since five years from 26 is 31, many people wait to apply for naturalization until they are 31 years old. An exception to the five-year moral standing role is if you are applying for naturalization under the circumstance that you are married or living with a U.S. citizen. In that case, you only need to show three years of good moral character.
Depending on your particular situation, you may consider waiting until either age 29 or 31 to apply for naturalization if you are 26 years old. Again, speak with your naturalization attorney about the best option. There are various ways to obtain citizenship without getting denied.