To apply to become a naturalized U.S. citizen, you must be at least 18 years of age. You also need to have had a valid Permanent Resident Card, typically referred to as a Green Card, for at least five years before applying. There are no age requirements for Green Card eligibility. 

To apply for a Green Card, you must show how you qualify to work or reside in the U.S. If you married a U.S. citizen in your home country, you may qualify to receive a Green Card. You may also receive a Green Card if a U.S. citizen adopted you before you turned 16. 

When can the citizenship application process begin? 

If unmarried, and one of your parents or stepparents is a U.S. citizen, you could apply for a Green Card while under the age of 21. After you turn 21, however, you may wish to consider applying for U.S. citizenship. The benefits of becoming a citizen include voting, running for public office and applying for a job with the federal government. 

As noted by the U.S. Citizenship and Immigration Services website, you must live in the state where you apply for at least three months. 

Does the application process include a test? 

Your application will include a test of your English reading, speaking and writing skills. The test also covers your understanding of civics, such as American history and the U.S. Constitution. An official generally asks 10 questions and you must orally answer at least six correctly. 

Can someone of advanced age apply for citizenship? 

Advanced age is not a disqualifying factor. A Texas resident, for example, applied for citizenship a few weeks before turning 103 years old and after living in the U.S. as a permanent resident for 30 years. After a successful naturalization process, she registered to vote, as reported by KHOU-TV.