There are three paths to become a U.S. citizen after the age of 18:
1) Be a permanent resident (green card holder) for over 5 years (Check the general requirements above).
2) Marry a citizen of the United States, but there are conditions:
<span>He or she has been a citizen of the U.S. for over 3 years.<span>Be a green card holder for at least 5 years since the filling of your application for naturalization (form N-400). Out of the 5 years, you have physically been in the States for at least 30 months! You cannot leave the United States for longer than 1 year. If you stay away longer than 6 months, you will have to prove that you did not abandon the United States of America.</span>You have been living in marital union for that timeYou both are 18 years or olderYou lived in one of the states for at least 3 months.You are fluent in English (in writing, reading and speaking) and you posses sufficient knowledge of the U.S. history and government.You have been a decent U.S. citizen. This includes living according to the laws of the Constitution of the United States.</span>
3) Join the military, here are some requirements as well:
<span>You are 18 years or olderYou have served honorably in the U.S. armed forces for at least 1 year and, if separated from the U.S. armed forces, have been separated honorably.You are fluent in English (in writing, reading and speaking) and you posses sufficient knowledge of the U.S. history and government.You have been a decent U.S. citizen. This includes living according to the laws of the Constitution of the United States.<span>You have lived in the United States for at least 5 years and lived physically in the United States for at least 30 months out of the 5 years immediately preceding the date of filing the application, unless you have filed an application while still in the service or within 6 months of separation.
hope this helps</span></span>
