The requirement to register with selective service ceases when a man reaches 26 years of age. Men who did not live in the United States between 18 and 26 years of age, and men who lived in the United States between 18 and 26 years of age, but maintained lawful nonimmigrant status for the entire period are not required to register.
The registration requirement for selective services is tied to the “Good Moral Character” requirement for naturalization.
For more information on the selective services go here.
For more information on citizenship requirements go here.